Method for passivating metal surfaces

Abstract

Method for passivating a metallic surface, said method comprising: - providing said metallic surface; - exposing said metallic surface to a modified Caro's acid composition selected from the group consisting of: composition A; composition B and Composition C; wherein said composition A comprises: - sulfuric acid in an amount ranging from 20 to 70 wt% of the total weight of the composition; - a modifier component comprising an amine moiety and a sulfonic acid moiety selected from the group consisting of: taurine; taurine derivatives; and taurine- related compounds; and - a peroxide; wherein said composition B comprises: - an alkylsulfonic acid: and - a peroxide; wherein the acid is present in an amount ranging from 40 to 80 wt% of the total weight of the composition and where the peroxide is present in an amount ranging from 10 to 40 wt% of the total weight of the composition; wherein said composition C comprises: - sulfuric acid; - a two-part modifier component comprising: - a compound comprising an amine moiety; and - a compound comprising a sulfonic acid moiety; and - a peroxide; for a period of time sufficient to coat said metallic surface with a metal oxide film created by the exposure of said metallic surface to said modified Caro's acid composition.

Description

[0001] METHOD FOR PASSIVATING METAL SURFACES

[0002] FIELD OF THE INVENTION

[0003] The present invention is directed to a novel method to passivate metals, more specifically it is directed to a method using a modified Caro’s acid to passivate metals.

[0004] BACKGROUND OF THE INVENTION

[0005] Metal passivation, in the fields of chemistry and engineering, refers to the generation of a coating on a material to render the latter less prone to being affected or corroded by its surrounding environment. This coating renders the surface of the material passive.

[0006] The coaling which renders the material passive is formed by a chemical reaction with the base material, or in some cases, it is formed by exposing the material to air so as to oxidize it. Generally, the coating involves a metal oxide which is quite unreactive and hence ideal for exposure to various solutions and environments. This layer of metal oxide provides protection against corrosion, which would decrease the material’s lifespan.

[0007] Metal passivation is a post-fabrication method of maximizing the inherent corrosion resistance of a stainless alloy from which the workpiece was produced. It is not a scale removal treatment, nor is it like a coat of paint. Passivation provides valuable corrosion resistance of parts and components machined from stainless steels. Properly conducted passivation can prevent premature failure of metals, but when not properly conducted, it may actually provide weak points for corrosion to begin.

[0008] Passivation can be best understood as a deliberate-controlled corrosion. Once in a passivation bath, the acid dissolves, or corrodes, free iron (or other alloy metals) at the surface of the metal. This is done in a controlled and uniform manner so long as the bath parameters, such as temperature, acid concentration and exposure time, are followed. When the reaction is not properly overseen, a runaway reaction of corrosion can occur and is referred in the industry as a “flash attack.” When a metal undergoes a flash attack, it can develop a dark and damaged or etched surface.

[0009] Proper passivation requires at least two steps: the cleaning of the metal followed by exposure of the metal to a passivation bath. Cleaning metals prior to exposing them to a passivation bath is a critical step, as various contaminants, however small they may be, can find their way onto a metallic surface and generate weak points on the metallic surface where corrosion may take hold. Contaminants such as grease, coolant or other shop debris must be thoroughly cleaned from the surface to obtain the best possible corrosion resistance. It is important when performing passivation of metals that a number of steps be taken beforehand. With respect to various metals which have been machined, machining chips must be removed, and this can be simply by wiping off the metal piece. A subsequent step can include the use of a commercial degreaser or cleaner to remove various oils or fluids used in the machining step

[0010] Should the oils not be removed or not be properly removed, one can expect the formation of gas bubbles on the surface of the metal being passivated. The presence of gas bubbles will impede the passivation and may create weak spots on the passivated metal piece. In some cases, an accumulation of chlorides can lead to a “flash attack” which leaves a heavily etched or darkened surface, in essence, damage to the surface which w as destined to be protected through passivation.

[0011] It becomes evident that proper steps need to be taken when understanding that these passivated metals arc used in many critical industries including, but not limited to, aerospace, medical and dental, and various other industries.

[0012] Passivating Baths

[0013] After thorough cleaning, the stainless-steel part is ready for immersion in a passivation acid bath. A number of conventional approaches are currently widespread in the field: nitric acid passivation, nitric acid with sodium dichromate passivation, and citric acid passivation.

[0014] The preferred passivation approach typically depends on the type of stainless steel and the acceptance criteria. Today, both nitric acid and citric acid passivation are widely used and relied upon in many industries. Both passivation approaches have shown effectiveness in passivating numerous types of stainless steel and are extensively described in the industry standards ASTM A967 and AMS 2700. It is worth noting that passivation of titanium is governed by ASTM F86, which only specifies passivation by exposure to nitric acid.

[0015] Stainless steels which can be passivated using citric acid at temperatures above 120°F, include: Austenitic: Martensitic -PH; Ferritic (type 430); Martensitic; and Austenitic-FM. Stainless steels which can be passivated using citric acid at temperatures below 120°F, include: Fcrritic-FM (Types 430F & 430FR); Ferritic-FM (Chrome Core® 18-FM); Ferritic-FM (Type 409Cb-FM); and Martensitic -FM (Type 416).

[0016] Stainless steels which can be passivated using nitric acid (typically at 20 vol.%) at temperatures above 120°F, include Austenitic stainless steel. Stainless steels which can be passivated using nitric acid (typically at 20 vol.%) at temperatures below 120°F but in the presence of sodium dichromate (Na2Cr2O7), include: Martcnsilic-PH; Ferritic (type 430); Martensitic; Auslenilic-FM; Ferritic-FM (Types 430F & 430FR); Ferritic-FM (Type 409Cb-FM); and Marlensitic-FM (Type 416)

[0017] Nitric Acid Passivation

[0018] Nitric acid is a mineral acid which is a hazardous chemical in that it emits toxic and corrosive fumes. It requires special attention to ventilation and safe chemical handling. It is highly reactive to skin and can rapidly cause severe chemical burns. Nitric acid emissions in the atmosphere have been associated with acid rain and smog and can even impact the ozone layer. These and other factors explain the high degree of oversight with respect to nitric acid and its handling and use, which translate into rigorous requirements for safety in handling and environmental protection.

[0019] Stainless steels which may be more prone to etching (flash attack) during the passivation in nitric acid can benefit from the addition of sodium dichromate passivating composition. Other options to reduce the risk of flash attacks during nitric acid passivation include the use of higher concentrations of nitric acid as well as higher bath temperatures. While these options to reduce flash attacks may work, they also increase the risk to personnel safety. It is worth noting that sodium bichromate is a hexavalent chromium compound, which is a known carcinogenic substance.

[0020] Citric Acid Passivation

[0021] In comparison to nitric acid passivation, citric acid passivation offers a generally safer and more environmentally friendly approach to passivation.

[0022] Citric acid is an organic acid which is safer for operators than nitric acid as it is low fuming and much less toxic than its nitric acid counterpart. Citric passivation is widely used as it meets current industry standards and is applicable to most types of stainless steel.

[0023] In the past, some manufacturers regarded citric acid passivation negatively due to the possible mould growth in the citric acid baths. Nowadays, citric acid passivation has overcome this perception, since some formulations now contain biocides aimed at preventing organic growth and mould. In fact, citric acid passivation has become increasingly popular with manufacturers who want to want to steer clear of the nitric acid compositions and especially those containing sodium dichromate. These compositions can be very difficult to dispose of and are subject to more regulations than citric acid. Citric acid is considered environmentally friendly, which explains its widespread acceptance.

[0024] Nonetheless, citric acid passivation requires close monitoring of immersion time, bath temperature and concentration in order to avoid “flash attack” which would otherwise mar or damage the metal being passivated.

[0025] In fact, laboratory tests have shown that passivation with citric acid was more likely to cause “flash attack” than passivation with nitric acid. Some of the reasons for the occurrence of flash attacks can be found in excessive bath temperature, excessive immersion time and contaminants present in the baths. Citric acid compositions used in metal passivation contain chemicals such as, but not limited to, corrosion inhibitors and other additives (such as wetting agents) that are said to minimize the occurrence of flash attacks.

[0026] It has been noted that another advantage of citric acid passivation is that it involves, in many cases, shorter passivation times versus nitric acid passivation.

[0027] When passivating metals using citric acid, it is important to monitor the temperature, the time of immersion and the concentration of the acid so as to avoid any possible runaway corrosion referred to as a flash attack. Some advantages of using citric acid passivation rather than nitric acid passivation include the following: no emission of toxic and corrosive gases during the process; lower handling requirements in terms of safety equipment; lower concentration of acid is required; lower overall cost to use; process can be earned out a room temperature in the case of some metals and without the need for ventilation; uses environmentally friendly chemicals, especially important when it comes time to dispose of spent acid; and the process does not lead to corrosion of the equipment employed. In terms of overall process stability, citric acid passivation is less prone to time and temperature variation than nitric acid passivation. Furthermore, citric acid passivation does not emit any hazardous vapors.

[0028] In light of the state-of-thc art, there exists a need for a new composition which can be used for the passivation of stainless steel which shows advantages not shared with conventional composition and methods associated therewith. The inventors have surprisingly and unexpectedly discovered a new acid composition and method to passivate metals using a composition which overcome some of the drawbacks of the known and conventionally used passivation methods.

[0029] SUMMARY OF THE INVENTION

[0030] According to an aspect of the present invention, there is provided a method for passivating a metallic surface, said method comprising:

[0031] - providing said metallic surface;

[0032] - exposing said metallic surface to a modified Caro’s acid composition selected from the group consisting of: composition A: composition B and Composition C : wherein said composition A comprises:

[0033] - sulfuric acid in an amount ranging from 20 to 70 wt% of the total weight of the composition:

[0034] - a modifier component comprising an amine moiety and a sulfonic acid moiety selected from the group consisting of: taurine; taurine derivatives; and taurine- related compounds; and

[0035] - a peroxide; wherein said composition B comprises:

[0036] - an alkylsulfonic acid: and

[0037] - a peroxide; wherein the acid is present in an amount ranging from 40 to 80 wt% of the total weight of the composition and where the peroxide is present in an amount ranging from 10 to 40 wt% of the total weight of the composition, wherein said composition C comprises:

[0038] - sul furic acid;

[0039] - a two-part modifier comprising:

[0040] - a compound comprising an amine moiety; and

[0041] - a compound comprising a sulfonic acid moiety; and

[0042] - a peroxide; for a period of time sufficient to coat said metallic surface with a metal oxide film created by the exposure of said metallic surface to said modified Caro’s acid composition.

[0043] According to a preferred embodiment of the present invention, said sulfuric acid, said compound comprising an amine moiety and a sulfonic acid moiety and said peroxide arc present in a molar ratio of no less than 1:1:1. Preferably, said sulfuric acid, said compound comprising an amine moiety and a sulfonic acid moiety and said peroxide arc present in a molar ratio of no more than 15:1 :1. According to another preferred embodiment of the present invention, said sulfuric acid and said compound comprising an amine moiety and a sulfonic acid moiety are present in a molar ratio of no less than 3:1.

[0044] Preferably, said compound comprising an amine moiety and a sulfonic acid moiety is selected from the group consisting of: taurine; taurine derivatives; and taurine-related compounds. According to a preferred embodiment of the present invention, said taurine derivative or taurine-related compound is selected from the group consisting of: taurolidine; taurocholic acid; tauroselcholic acid; tauromustinc; 5- taurinomethyluridine and 5-taurinomethyl-2-thiouridine; homotaurine (tramiprosate); acamprosate; and laurates; as well as aminoalkylsulfonic acids where the alkyl is selected from the group consisting of C1-C5 linear alkyl and C1-C5 branched alkyl. Preferably, said linear alkylaminosulfonic acid is selected form the group consisting of: methyl; ethyl (taurine); propyl; and butyl. Also preferably, said branched aminoalkylsulfonic acid is selected from the group consisting of: isopropyl; isobutyl; and isopentyl. More preferably, said compound comprising an amine moiety and a sulfonic acid moiety' is taurine. According to a preferred embodiment of the present invention, said alkylsulfonic acid is selected from the group consisting of: methanEsulfonic acid; ethancsulfonic acid; propanesulfonic acid; butanesulfonic acid; pentanesulfonic acid; hexanesulfonic acid; and combinations thereof.

[0045] According to a preferred embodiment of the present invention, said sulfuric acid and compound comprising an amine moiety and a sulfonic acid moiety arc present in a molar ratio of no less than 3:1 .

[0046] According to another aspect of the present invention, there is provided a method for passivating a metall ic surface, said method comprising:

[0047] - providing said metallic surface;

[0048] - exposing said metallic surface to a composition comprising:

[0049] - an alkyl sulfonic acid; and

[0050] - a peroxide; wherein the acid is present in an amount ranging from 40 to 80 wt% of the total weight of the composition and where the peroxide is present in an amount ranging from 10 to 40 wi% of the total weight of the composition.

[0051] Preferably, the method uses an aqueous acidic composition comprising:

[0052] - an alkyl sulfonic acid; and

[0053] - a peroxide; wherein said alkylsulfonic acid, and said peroxide are present in a molar ratio of no less than 1:1. Preferably, said alkylsulfonic acid is selected from the group consisting of: metlianesulfonic acid: ethanesulfonic acid; propancsulfonic acid; butancsulfonic acid; pentanes ulfonic acid; hexanesulfonic acid; and combinations thereof. More preferably, said alkylsulfonic acid is methanesulfonic acid.

[0054] According to a preferred embodiment of the present invention, the method further comprises a compound comprising an amine moiety. Preferably, the compound comprising an amine moiety is a primary amine. More preferably, the compound comprising an amine moiety is an alkanolamine. More preferably, said alkanolamine is selected from the group consisting of: monoethanolamine; diethanolamine; triethanolamine; and combinations thereof.

[0055] According to a preferred embodiment of the present invention, the compound comprising an amine moiety is a tertiary amine. More preferably, said alkanolamine is triethanolamine.

[0056] According to an aspect of the present invention, there is provided a method for passivating a metallic surface, said method comprising;

[0057] - providing said metallic surface;

[0058] - exposing said metallic surface to a composition comprising;

[0059] - a compound comprising an amine moiety;

[0060] - a compound comprising a sulfonic acid moiety; and

[0061] - a peroxide; for a period of time sufficient to coat said metallic surface with a metal oxide film, wherein the acid is present in an amount ranging from 40 to SO wt% of the total weight of the composition and where the peroxide is present in an amount ranging from 10 to 40 wt% of the total weight of the composition. Preferably, the sulfuric acid and said a compound comprising an amine moiety and said compound comprising a sulfonic acid moiety are present in a molar ratio of no less than 1:1:1. sulfuric acid, said compound comprising an amine moiety and said compound comprising a sulfonic acid moiety are present in a molar ratio ranging from 28: 1 : 1 to 2:1: 1. According to a preferred embodiment of the present invention, the compound comprising an amine moiety has a molecular weight below 300 g / mol.

[0062] According to a preferred embodiment of the present invention, the compound comprising a sulfonic acid moiety is selected from the group consisting of: alkylsulfonic acids and combinations thereof. Preferably, said alkylsulfonic acid is selected from the group consisting of: alkylsulfonic acids where the alkyl groups range from C1-C6 and arc linear or branched: and combinations thereof.

[0063] DETAILED DESCRIPTION OF THE IN VENTION

[0064] It will be appreciated that numerous specific details have been provided for a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In. other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. Furthermore, this description is not to be considered so that it may limit the scope of the embodiments described herein in any way, but rather as merely describing the implementation of the various embodiments described herein.

[0065] According to a preferred embodiment of the present invention, there is provided a method for the passivation of a steel surface, wherein said method comprises the steps of:

[0066] - providing said steel surface to be passivated;

[0067] - cleaning said steel surface to remove oil and other contaminants:

[0068] - exposing said steel surface to a modified Caro’s acid for a period of time sufficient to form a layer of metal oxide thereon, wherein said modified Caro’s acid composition selected from the group consisting of: composition A; composition B; and Composition C; wherein said composition A comprises:

[0069] - sulfuric acid in an amount ranging from 20 to 70 wt% of the total weight of the composition;

[0070] - a modifier component comprising an amine moiety and a sulfonic acid moiety selected from the group consisting of: taurine; taurine derivatives; and taurine- related compounds; and

[0071] - a peroxide; wherein said composition B comprises:

[0072] - an alkylsulfonic acid; and - a peroxide; wherein the acid is present in an amount ranging from 40 to 80 wt% of the total weight of the composition and where the peroxide is present in an amount ranging from 10 to 40 wt% of the total weight of the composition; wherein said composition C comprises:

[0073] - sulfuric acid;

[0074] - a two-part modifier comprising:

[0075] - a compound comprising an amine moiety; and

[0076] - a compound comprising a sulfonic acid moiety; and

[0077] - a peroxide; for a period of time sufficient to coat said metallic surface with a metal oxide film created by the exposure of said metallic surface to said modified Caro’s acid composition.

[0078] According to a preferred embodiment of the present invention, said sulfuric acid, said compound comprising an amine moiety and a sulfonic acid moiety and said peroxide are present in a molar ratio of no less than 1: 1:1. Preferably, said sulfuric acid, said compound comprising an amine moiety and a sulfonic acid moiety and said peroxide are present in a molar ratio of no more than 15:1:1. Preferably also, said sulfuric acid and said compound comprising an amine moiety and a sulfonic acid moiety are present in a molar ratio of no less than 3: 1.

[0079] According to a preferred embodiment of the present invention, said compound comprising an amine moiety and a sulfonic acid moiety is selected from the group consisting of: taurine; taurine derivatives; and taurine-related compounds. Preferably, said taurine derivative or taurine-related compound is selected from the group consisting of: taurolidine; taurocholic acid; tauroselcholic acid; tauromustine; 5- taurinomethyluridine and 5-taurinomethyl-2-thiouridine; homotaurine (tramiprosate); acamprosate; and laurates; as well as aminoalkylsulfonic acids where the alkyl is selected from the group consisting of C1-C5 linear alkyl and C1-C5 branched alkyl. Preferably, said linear aikylaminosul Tonic acid is selected from the group consisting of: methyl; ethyl (taurine); propyl; and butyl. Preferably, said branched aminoalkylsulfonic acid is selected from the group consisting of: isopropyl; isobutyl; and isopentyl. Most preferably, said compound comprising an amine moiety and a sulfonic acid moiety is taurine.

[0080] According to a preferred embodiment of the present invention, said compound comprising an amine moiety is an alkanolamine is selected from the group consisting of: monoethanolamine; diethanolamine; triethanolamine; and combinations thereof. According to a preferred embodiment of the present invention, said compound comprising a sulfonic acid moiety is selected from the group consisting of: alkylsulfonic acids and combinations thereof. Preferably, said alkylsulfonic acid is selected from the group consisting of: alkylsulfonic acids where the alkyl groups range from C1-C6 and are linear or branched; and combinations thereof. More preferably, said alkylsulfonic acid is selected from the group consisting of: methanesulfonic acid; ethanesulfonic acid; propanesulfonic acid; 2 -propanes ulfonic acid; isobulylsulfonic acid; t-butylsulfonic acid; butanesulfonic acid; iso-pcntylsulfonic acid; l-pcntylsulfonic acid; pentanesulfonic acid; t-bulylhexancsulfonic acid; and combinations thereof More preferably, said compound comprising a sulfonic acid moiety is methane sulfoni c acid.

[0081] According to a preferred embodiment of the present invention, the step of exposing said metallic surface to a modified Caro’s acid is carried out for a duration of 24 hours, preferably it is carried out for 18 hours, more preferably it is carried out for 12 hours, more preferably it is carried out for 10 hours, more preferably it is carried out for 9 hours, more preferably it is carried out for 8 hours, more preferably it is carried out for 6 hours, more preferably it is carried out for 6 hours, more preferably it is carried out for 4 hours, more preferably it is carried out for 3 hours, more preferably it is carried out for 2 hours, and even more preferably it is carried out for 1 hour. According to a preferred embodiment of the present invention, the step of exposing said metallic surface to a modified Caro’s acid is carried out for a duration of 45 minutes, more preferably it is carried out for 30 minutes, even more preferably for 15 minutes.

[0082] According to a preferred embodiment of the present invention, the step of exposing said metallic surface to a modified Caro’s acid is carried out at a temperature of 50°C, more preferably it is carried out at a temperature of 45°C, more preferably it is carried out at a temperature of 40°C, more preferably it is carried out at a temperature of 35°C, more preferably it is carried out at a temperature of 30°C, more preferably it is carried out at a temperature of 25°C, and yet even more preferably at room temperature.

[0083] According to a preferred embodiment of the present invention, the step of exposing said metallic surface to a modified Caro’s acid is preceded by a thorough cleaning. Preferably, the cleaning step will remove various contaminants such as grease, coolant or machining debris.

[0084] Preferably, the cleaning step can be followed by the application of a commercial degreaser or cleaner to remove various oils or fluids used in the machining step which involves the metal to be passivated. According to a preferred embodiment of the present invention, the step of exposing said metallic surface to a modified Caro’s acid is followed by a rinsing step where the metal passivated is rinsed at least once to remove any contaminants and acid which may be present on the surface thereof.

[0085] According to a preferred embodiment of the present invention, the metal surface once passivate offers a very smooth surface substantially or completely free of pits.

[0086] According to a preferred embodiment of the present invention, there is provided a method for the passivation of a steel surface, wherein said method comprises the steps of:

[0087] - providing said steel surface to be passivated;

[0088] - cleaning said steel surface to remove oil and other contaminants;

[0089] - exposing said steel surface to a modified Caro’s acid for a period of time sufficient to form a layer of metal oxide thereon, wherein said modified Caro’s acid composition selected from the group consisting of: composition A; composition B and Composition C; wherein said composition A comprises:

[0090] - sulfuric acid in an amount ranging from 20 to 70 wt% of the total weight of the composition;

[0091] - a modifier component comprising an amine moiety and a sulfonic acid moiety selected from the group consisting of: taurine; taurine derivatives; and taurine- related compounds; and

[0092] - a peroxide; wherein said composition B comprises:

[0093] - an alkyl sulfonic acid; and

[0094] - a peroxide; wherein the acid is present in an amount ranging from 40 to 80 wt% of the total weight of the composition and where the peroxide is present in an amount ranging from 10 to 40 wt% of the total weight of the composition; wherein said composition C comprises:

[0095] - sulfuric acid;

[0096] - a two-part modifier comprising;

[0097] - a compound comprising an amine moiety; and

[0098] - a compound comprising a sulfonic acid moiety; and

[0099] - a peroxide; for a period of time sufficient to coat said metallic surface with a metal oxide film created by the expos ure of said metallic surface to said modified Caro’s acid composition.

[0100] According to a preferred embodiment of the present invention, said alkylsulfonic acid; and said peroxide arc present in a molar ratio of no less than 1:1.

[0101] According to a preferred embodiment of the present invention, in Composition C, said sulfuric acid and said compound comprising an amine moiety and said compound comprising a sulfonic acid moiety arc presen t in a molar ratio of no less than 1:1 :1. Preferably, the sulfuric acid and said a compound comprising an amine moiety are present in a molar ratio of no less than 1:1. Preferably, the sulfuric acid and said compound comprising an amine moiety are present in a molar ratio of no less than 5:1. Preferably, the sulfuric acid and said compound comprising an amine moiety are present in a molar ratio of no less than 10:1. Preferably, the sulfuric acid and said compound comprising an amine moiety are present in a molar ratio of no less than 15: 1. Preferably, the sulfuric acid and said a compound comprising an amine moiety are present in a molar ratio of no less than 20:1. Preferably, the sulfuric acid and said compound comprising a sulfonic acid moiety are present in a molar ratio of no less than 1 :1. Preferably, the sulfuric acid and said compound comprising a sulfonic acid moiety are present in a molar ratio of no less than 5:1. Preferably, the sulfuric acid and said compound comprising a sulfonic acid moiety are present in a molar ratio of no less than 10:1. Preferably, the sulfuric acid and said compound comprising a sulfonic acid moiety arc present in a molar ratio of no less than 15:1. Preferably, the sulfuric acid and said compound comprising a sulfonic acid moiety arc present in a molar ratio of no less than 20:1.

[0102] According to a preferred embodiment of the present invention, in Composition C, said sulfuric acid, said compound comprising an amine moiety and said compound comprising a sulfonic acid moiety are present in a molar ratio ranging from 28:1: 1 to 2: 1 : ! . Preferably, the Composition C is a modified Caro’s acid comprising a sulfuric acid, hydrogen peroxide methanesulfonic acid and triethanolamine at a molar ratio of 10:10:1:1.

[0103] According to a preferred embodiment of the present invention, the metallic surface to be passivated is stainless steel.

[0104] According to a preferred embodiment of the present invention, the metallic surface to be passivated is a stainless steel selected from the group consisting of: Austenitic; Martensitic-PH; Ferritic; Martensitic; Austcnitic-FM; Ferritic-FM; and Martensitic-FM. According to a preferred embodiment of the present invention, the metallic surface to be passivated is Austenitic stainless steel, having a chromium content ranging from 15.0 to 23.5 %.

[0105] According to a preferred embodiment of the present invention, the metallic surface to be passivated is Austenitic stainless steel and is selected from the group consisting of; Type 304 / 304L; Type 316 / 316L; Type 305; Custom Flo 302HQ.

[0106] According to a preferred embodimen t of the present invention, the metallic surface to be passivated is a Martensitic-PH stainless steel having a chromium content ranging from 1 1.0 to 17.5 %.

[0107] According to a preferred embodiment of the present invention, the metallic surface to be passivated is Martensitic-PH stainless steel and is selected from the group consisting of: Custom 630 (17Cr-4Ni); Custom 450: Custom 455; Custom 465T; and 15Cr-5Ni.

[0108] According to a preferred embodiment of the present invention, the metallic surface to be passivated is a Martensitic stainless steel, having a chromium content of less than 15 %.

[0109] According to a preferred embodiment of the present invention, the metallic surface to be passivated is Martensitic stainless steel and is selected from the group consisting of: Type 410; Type 420; and TrimRitc®.

[0110] According to a preferred embodiment of the present invention, the metallic surface to be passivated is a Ferritic stainless steel having a chromium content above 16 %. Preferably, said ferritic stainless steel is type 430.

[0111] According to a preferred embodiment of the present invention, the metallic surface to be passivated is a Ferritic stainless steel having a chromium content of less than 12 %. Preferably, said ferritic stainless steel is type 409.

[0112] According to a preferred embodiment of the present invention, the metallic surface to be passivated is a Austenitic-FM stainless steel having a chromium content ranging from 17 to 19 %. Preferably, said Austenitic-FM stainless steel is type 303. According to a preferred embodiment of the present invention, the metallic surface to be passivated is a Ferritic-FM stainless steel having a chromium content above 16 %, Preferably, said Ferritic-FM stainless steel is selected from the group consisting of: type 430F; 430FR; and Chrome Core* 18-FM,

[0113] According to a preferred embodiment of the present invention, the metallic surface to be passivated is a Ferritic-FM stainless steel having a chromium content of less than 13 %. Preferably, said Ferritic-FM stainless steel is type 409Cb-FM.

[0114] According to a preferred embodiment of the present invention, the metallic surface to be passivated is a Martensitic-FM stainless steel having a chromium content of less than 13 %. Preferably, said Martensitic-FM stainless steel is type 416.

[0115] Conventional nitric acid baths are changed when, upon titration, it is deemed that the solution is no longer capable of providing proper passivation. Another indicator used to decide whether to discard the acidic solution is by visual inspection and assessing the color of the bath.

[0116] According to a preferred embodiment of the present invention, the passivation bath using a modified Caro’s acid, is able to process more metal than a conventional passivation bath. Moreover, it can extend its useful lifetime (i.e., period of time until the bath chemicals need to be discarded and replaced) by adding the peroxide component at intervals where the bath is known to start to lose its passivation efficiency.

[0117] According to a preferred embodiment of the present invention, there is provided a kit to passivate metals, wherein said kit comprises instructions for admixing the chemicals to form a modified Caro’s acid as well as at least one of the three components to prepare said modi fied Caro’s acid along with instructions to follow for passivating a metal or metallic surface.

[0118] Experiments to passivate various steels

[0119] Various steel coupons were exposed to a composition comprising a modified Caro’s acid as set out previously in order to assess the resulting passivated coupon’s ability to withstand corrosion through acid exposure.

[0120] Corrosion of a Modified Caro’s acid to treat Stainless Steel The purpose of this experiment is to test the corrosion rates of stainless steel in the presence of a modified Caro’s acid and the corrosion rates of stainless steel after exposure to the modified Caro’s acid composition.

[0121] Procedure

[0122] Corrosion tests were completed in glass sample jars in a healed water bath. For each condition listed in Tabic 1, the coupon was washed with acetone, air dried, and weighed, before being suspended in the test fluid The fluid in each glass sample jar was pre-heated to temperature before exposing the coupon to the acid blend. After the exposure period, the coupon was removed, washed with water, followed by an acetone wash, air dried, and then weighed. The corrosion rate was determined from the weight loss, and the pitting index (see Pitting index scale found in Finsgar, M.; Jackson, J. Corrosion Science, 2014, 86, 17—41 ) was evaluated visually at 40X magnification, and a photo of the coupon surface at 40X magnification was taken.

[0123] Table 1 lists the various compositions which were tested in the corrosion testing. The corrosion testing was carried out for a duration of 24 hours al a temperature of 30° C (86°F) under atmospheric pressure. The modified acid used in the passivation of metal coupons consisted of a modified Caro’s acid comprising a sulfuric acid, hydrogen peroxide methanesulfonic acid and triethanolamine at a molar ratio of 10:10: 1 :1.

[0124] Table 1: Listing of the compositions and coupons tested

[0125] The corrosion test results are shown Table 2, and in Figure. 1 to Figure 2. Figure 1: Surface of 3O4SS coupon from corrosion test B-2 at 40X Magnification. Figure 2: Surface of 316SS coupon from corrosion test C-2 at 40X Magnification. The various coupons exposed to the various listed compositions for periods of time of 24 hours at varying temperatures. A preferable result is one where the lb / ft2corrosion number is at or below 0.05. More preferably, that number is at or below 0.02. Table 2: Results of the corrosion testing of treated and untreated coupons at 30°C under atmospheric pressure for a duration of 24 hours

[0126] Conclusion

[0127] From the above experiments, it was found at the tested conditions that the modified Caro’s acid composition was not corrosive to 304SS and 316SS (coupons B-l and C-l). Moreover, exposure of coupons B- 1 and C-1 to a modified Caro’s acid according to a preferred method of the present invention clearly passivated their surfaces. This is confirmed by subsequent exposure of those coupons to 70 wt% which resulted in an unblemished surface (as indicated by the photographs taken of the coupons now labelled B-2 and C-2).

[0128] Moreover, after the exposure to the modified Caro’s acid, the steel coupons of 304SS and 316SS were resistant to sulfuric acid corrosion at the tested conditions, whereas coupons not exposed to the modified Caro’s acid were heavily corroded by sulfuric acid.

[0129] High temperature testing

[0130] In order to assess the efficiency and the limits of the compositions used for metal passivation, experiments were carried out using various higher temperature exposing the various coupons to 70% sulfuric acid for a period of 24 hours and at temperature of 45°C, 60°C and 75°C.

[0131] Table 3: Results of the higher temperature corrosion testing of treated and untreated coupons at 45°C under atmospheric pressure for a duration of 24 hours fable 4: Results of the higher temperature corrosion testing of treated and untreated coupons at 60°C under atmospheric pressure for a duration of 24 hours

[0132] Table 5: Results of the higher temperature corrosion testing of treated and untreated coupons at 75°C under atmospheric pressure for a duration of 24 hours

[0133] In light of the results for in the higher temperature testing as set out in tables 3, 4 and 5 (above), it can be said that metal passivation, using a composition according to a preferred embodiment of the present invention, provides effective protection against corrosion from exposure to concentrated sulfuric acid at a temperature of up to 45°C. The corrosion protection obtained in the testing at 60°C showed some good protection despite somewhat failing to maintain the corrosion under the threshold of 0.05 lb / ft2, especially in the case of the 304 stainless steel. The 70°C testing series showed the limits of what metal passivation can achieve. It is understood that at such temperatures a metal exposed to 70 % sulfuric acid would require the latter comprise some form of corrosion inhibitor to reduce corrosion.

[0134] Upon reviewing the results, it is observed that the corrosion under the tested conditions indicate a close to linear behavior which would indicate that the highest temperature where the corrosion is still acceptable would seem to range between 50°C and 52.5°C. It is worth noting that the conditions selected were deliberately harsh in order to exhibit the quality of the passivation obtained. A sulfuric acid composition of 70% is known to be extremely corrosive, due to the water / acid ratio. At more diluted sulfuric acid concentrations, it is expected that the higher temperature would come within the acceptable limits set out above.

[0135] Table 6: Results of the higher temperature corrosion testing of treated and untreated coupons at 45°C under atmospheric pressure for a duration of 24 hours using 10% sulfuric acid Table 7: Results of the higher temperature corrosion testing of treated and untreated coupons at 60°C under atmospheric pressure for a duration of 24 hours using 10% sulfuric acid

[0136] Table 8: Results of the higher temperature corrosion testing of treated and untreated coupons at 75°C under atmospheric pressure for a duration of 24 hours using 10% sulfuric acid

[0137] In light of the results for in the higher temperature testing as set out in tables 6, 7 and 8 (above), it can be said that metal passivation, using a composition according to a preferred embodiment of the present invention, provides effective protection against corrosion from exposure to dilute sulfuric acid at a temperature of up to 75°C. The corrosion protection obtained for each one of the coupons treated by passivation according to a preferred method of the present invention maintained the corrosion of the coupon under the desired threshold of 0.05 lb / ft2.

[0138] According to a preferred embodiment of the present invention, the compositions used for passivation of metals such as stainless steel can be reused be simply adding more peroxide. When used for passivating metals, the compositions consume the peroxide component and thus can be re-used a great number of limes without losing effectiveness, as the consumable peroxide can be replenished by simply adding it to the used composition. This is a significant advantage over standard nitric acid and citric acid solutions, as they lose effectiveness over time and must be disposed of entirely. According to a preferred embodiment of the present invention, the passivation method is employed to replace passivation by citric acid and treats metals used in a wide variety of industries, including but not limited to aerospace, medical, industrial and manufacturing industries.

[0139] While the foregoing invention has been described in some detail for purposes of clarity and understanding, it will be appreciated by those skilled in the relevant arts, once they have been made familiar with this disclosure that various changes in form and detail can be made without departing from the true scope of the invention in the appended claims

Claims

CLAIMS1. Method for passivating a metallic surface, said method comprising:- providing said metallic surface;- exposing said metallic surface to a modified Caro’s acid composition selected from the group consisting of: composition A: composition B and Composition C; wherein said composition A comprises:- sulfuric acid in an amount ranging from 20 to 70 wt% of the total weight of the composition;- a modifier component comprising an amine moiety and a sulfonic acid moiety selected from the group consisting of: taurine; taurine derivatives; and taurine- related compounds; and- a peroxide; wherein said composition B comprises:- an alkylsulfonic acid; and- a peroxide; wherein the acid is present in an amount ranging from 40 to 80 wt% of the total weight of the composition and where the peroxide is present in an amount ranging from 10 to 40 wt% of the total weight of the composition; wherein said composition C comprises:- sulfuric acid;- a two-part modifier comprising:- a compound comprising an amine moiety; and- a compound comprising a sulfonic acid moiety; and- a peroxide; for a period of time sufficient to coat said metallic surface with a metal oxide film created by the exposure of said metallic surface to said modified Caro’s acid composition.

2. The method according to claim 1 wherein said sulfuric acid, said compound comprising an amine moiety and a sulfonic acid moiety and said peroxide are present in a molar ratio of no less than 1 :1:1.

3. The method according to claim 1 or 2, wherein said sulfuric acid, said compound comprising an amine moiety and a sulfonic acid moiety and said peroxide are present in a molar ratio of no more than 15: 1 ;] .

4. The method according to any one of claims 1 to 3, wherein sulfuric acid and said compound comprising an amine moiety and a sulfonic acid moiety are present in a molar ratio of no less than 3: 1.

5. The method according to any one of claims 1 to 4, where said compound comprising an amine moiety and a sulfonic acid moiety is selected from the group consisting of: taurine; taurine derivatives; and taurine-related compounds.

6. The method according to any one of claims I to 5, where said taurine derivative or taurine-related compound is selected from the group consisting of: taurolidine; taurocholic acid; tauroselcholic acid; tauromustine; 5-taurinomethyluridine and 5-taurinomethyl-2-thiouridine; homotaurine (tramiprosate); acamprosate; and taurates: as well as aminoalkylsulfonic acids where the alkyl is selected from the group consisting of C1-C5 linear alkyl and C1-C5 branched alkyl.

7. The method according to any one of claims 1 to 6, where said linear alkylaminosulfonic acid is selected form the group consisting of: methyl; ethyl (taurine); propyl; and butyl.

8. The method according to claim 7, where said branched aminoalkylsulfonic acid is selected from the group consisting of: isopropyl; isobutyl; and isopentyl.

9. The method according to any one of claims 1 to 8, where said compound comprising an amine moiety and a sulfonic acid moiety is taurine.

10. The method according to any one of claims 1 to 9, wherein said sulfuric acid and compound comprising an amine moiety and a sulfonic acid moiety are present in a molar ratio of no less than 3:1.

11. Method for passivating a metallic surface, said method comprising:- providing said metallic surface;- exposing said metallic surface to a composition comprising:- an alkylsulfonic acid; and- a peroxide; wherein the acid is present in an amount ranging from 40 to 80 wt% of the total weight of the composition and where the peroxide is present in an amount ranging from 10 to 40 wt% of the total weight of the composition.

12. The method according to claim 1 1 wherein said aqueous acidic composition comprises:- an alkylsulfonic acid; and- a peroxide; wherein said alkylsulfonic acid; and said peroxide arc present in a molar ratio of no less than 1 : 1.

13. The method according to any one of claims 11 to 12 further comprising a compound comprising an amine moiety.

14. The method according to any one of claims 1 1 to 13, where said alkylsulfonic acid is selected from the group consisting of: methanesulfonic acid; ethanesulfonic acid; propanesulfonic acid; butanesiilfonic acid; pentanesulfonic acid; hexanesulfonic acid; and combinations thereof.15 The method according to any one of claims 1 1 to 14, where said alkylsulfonic acid is methane sulfonic acid.

16. The method according to claim 13, wherein the compound comprising an amine moiety is a primary amine.

17. The method according to claim 16, wherein the compound comprising an amine moiety is an alkanolamine.

18. The method according to claim 16, wherein the compound comprising an amine moiety is a tertiary’ amine.

19. The method according to claim 17, wherein said alkanolamine is selected from the group consisting of: monoethanolamine; diethanolamine; triethanolamine; and combinations thereof.

20. The method according to claim 19, wherein said alkanolamine is triethanolamine.

21. Method for passivating a metallic surface, said method comprising:- providing said metallic surface;- exposing said metallic surface to a composition comprising;- a compound comprising an amine moiety;- a compound comprising a sulfonic acid moiety; and- a peroxide;for a period of time sufficient to coat said metallic surface with a metal oxide film, wherein the acid is present in an amount ranging from 40 to RO wt% of the total weight of the composition and where the peroxide is present in an amount ranging from 10 to 40 wt% of the total weight of the composition.

22. The method according to claim 21, wherein sulfuric acid and said a compound comprising an amine moiety and said compound comprising a sulfonic acid moiety arc present in a molar ratio of no less than 1:1 : 1.

23. The method according to any one of claims 21 or 22, wherein sulfuric acid, said compound comprising an amine moiety and said compound comprising a sulfonic acid moiety are present in a molar ratio ranging from 28: 1:1 to 2:1:1.

24. The method according to any one of claims 21 to 23, wherein said compound comprising an amine moiety has a molecular weight below 300 g / mol.

25. The method according to any one of claims 21 to 24, wherein where said compound comprising an amine moiety is a primary amine.

26. The method according to any one of claims 21 to 25, wherein said compound comprising an amine moiety is an alkanolamine.

27. The method according to any one of claims 21 to 26, wherein said compound comprising an amine moiety is a tertiary amine.

28. The method according to any one of claims 21 to 27, wherein said alkanolamine is selected from the group consisting of: monoethanolamine; diethanolamine; triethanolamine; and combinations thereof29. The method according to any one of claims 21 to 28, wherein said alkanolamine is triethanolamine.

30. The method according to any one of claims 21 to 29, wherein said compound comprising a sulfonic acid moiety is selected from the group consisting of: alkylsulfonic acids and combinations thereof.31 The method according to any one of claims 21 to 30, wherein said alkylsulfonic acid is selectedfrom the group consisting of: alkylsulfonic acids where the alkyl groups range from C1-C6 and arc linear or branched; and combinations thereof.

32. The method according to claim 31, wherein said alkylsulfonic acid is selected from the group consisting of: methancsulfonic acid; cthancsulfonic acid; propancsulfonic acid, 2-propancsulfonic acid; isobutylsulfonic acid; t-butylsulfonic acid, butanesulfonic acid; iso-pentylsulfonic acid; t-pentylsulfonic acid; pcnlanesulfonic acid; t-bulylhcxanesulfonic acid; and combinations thereof.

33. The method according to any one of claims 21 to 32, wherein said compound comprising a sulfonic acid moiety is mcthancsulfonic acid.

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