Stress Corrosion Cracking Test Considering Container Environment

Abstract

A method for evaluating ammonia stress corrosion cracking (SCC) is proposed. The method comprises bending a specimen by applying a stress greater than a predetermined strength to the specimen using a Four Point Bending Test (FPBT); immersing the specimen for a predetermined period in a liquid or gaseous ammonia environment with predetermined flow conditions; and inspecting the stress corrosion cracking (SCC) of the specimen using an optical microscope.

Description

Technology Field

[0001] The following description relates to the evaluation of stress corrosion cracking (SCC), specifically to a method for performing SCC evaluation in a test apparatus simulating an ammonia cargo tank environment. Background Technology

[0002] With the recent rise in demand for measures against global warming, ammonia, which does not emit carbon dioxide during combustion, is attracting attention as a next-generation energy source. Furthermore, as ammonia can function as a carrier for hydrogen, which is difficult to transport and store, demand for it is increasing. Accordingly, research and development on ammonia storage and transport technologies, particularly regarding the construction of large ammonia tanks made of high-strength steel, is being continuously conducted.

[0003] Meanwhile, ammonia is corrosive to steel, and stress corrosion cracking (SCC) can occur in steel exposed to liquid or gaseous ammonia. Therefore, it is essential to verify the SCC resistance characteristics of steel in which ammonia is stored.

[0004] Figure 1 is a diagram illustrating a device used in the evaluation method for determining the suitability of metal materials in an ammonia environment by the IMO.

[0005] The International Maritime Organization (IMO) specifies the Four-Point Bending Test (FPBT) as an evaluation method for determining the suitability of metal materials in an ammonia environment. Figure 110 of Fig. 1 shows a bender to provide bending force in ammonia, and Figure 120 shows a jig to fix the specimen.

[0006] Specifically, the FPBT stipulated by the IMO specifies the following conditions.

[0007] 1) Specimen types: Base material and weldment

[0008] 2) Applied stress: Applied at 100% of the material's yield strength

[0009] 3) Environment: Liquid / gaseous ammonia conditions

[0010] 4) Evaluation Period: 30 days

[0011] 5) Check for SCC occurrence after the above test

[0012] type temperature Ammonia composition Liquid -33.5℃ 0.1 wt.% H2O + 2.5 ppm O2+ NH3bal. 2.5 ppm O2+ NH3bal. Gas Ambient Pure NH3(≥ 99.99 %) (7N) 0.9 % volume O2+ NH3bal.

[0013] However, the evaluation method under the IMO is not considered an evaluation method capable of determining material conformity because SCC does not occur even when IGC standards are exceeded.

[0014] Specifically, as a result of research conducted by the applicant in 2023, it was confirmed that no SCC occurred in steels of Act. YS 388 to 835 MPa under IMO test conditions. Although the test was evaluated based on IMO regulations, it was confirmed that no ammonia SCC occurred in any of the specimens, even though the yield strengths of E500 and PFS700 in [Table 2] below showed a significant difference from IGC regulations.

[0015] [Table 2]

[0016]

[0017] Accordingly, there is a need for an evaluation method capable of simulating the actual cargo hold environment.

[0019] In addition, to prevent SCC in ammonia cargo tanks, restrictions are placed not only on the ammonia composition (oxygen, hydrogen) but also on the yield strength (YS) and Ni content of the cargo tank steel; however, the technical basis for these restriction regulations appears to be insufficient.

[0020] Due to the upper limit of steel yield strength and restrictions on Ni content, only a very limited number of steels can be applied when selecting cargo tank materials, which has the disadvantage of incurring additional purchasing costs and making it impossible to handle various fuels. The problem to be solved

[0021] In one aspect of the present invention for solving the problem described above, a method for performing SCC evaluation using FPBT in a test apparatus simulating an actual ammonia cargo tank environment is proposed.

[0022] In addition, according to the embodiments, we intend to explain in detail how to simulate the flow conditions inside the cargo tank, how to configure the applied stress conditions, and how to set the temperature conditions of the ammonia environment.

[0023] The problems to be solved by the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which the present invention belongs from the description below. means of solving the problem

[0024] In one aspect of the present invention for solving the problem described above, a method for evaluating ammonia stress corrosion cracking (SCC) is proposed, comprising: applying a stress greater than a predetermined strength to a specimen using a Four Point Bending Test (FPBT) to bend the specimen; immersing the specimen for a predetermined period in a liquid or gaseous ammonia environment with predetermined flow conditions; and inspecting the stress corrosion cracking (SCC) of the specimen using an optical microscope.

[0025] The above predetermined flow conditions can correspond to the reciprocating motion conditions of an experimental device that takes into account the flow conditions inside the cargo tank.

[0026] Preferably, the above reciprocating motion condition may require at least one reciprocating motion per 3 seconds.

[0027] The stress greater than the above predetermined strength can correspond to a stress greater than the yield stress at room temperature.

[0028] Preferably, the stress greater than or equal to the above-mentioned predetermined strength can correspond to a stress within the range greater than or equal to the yield stress and less than or equal to the tensile stress.

[0029] The above ammonia environment is -33.5 o C or higher 45 o It is desirable to set the temperature conditions to be C or lower.

[0030] The above specimen includes a base material and a weldment specimen, and the above predetermined period of immersion in the ammonia environment may correspond to a period of 30 days or more.

[0031] Preferably, the above-mentioned predetermined period may be set to a period of 30 days or more and 120 days or less, and in one embodiment, it may be set to a period of 90 days.

[0032] Immersing the specimen in the liquid ammonia environment may include: loading the specimen, which has been bent using the FPBT, into a pressure vessel; performing argon purging to remove air inside the pressure vessel; purging the pressure vessel using a mixture of oxygen and argon in a predetermined ratio; opening the discharge valve of the pressure vessel and loading the liquid ammonia so that the specimen is submerged in the liquid ammonia; cooling the internal temperature of the pressure vessel to a predetermined temperature; and loading the mixture of oxygen and argon in a predetermined ratio so that the internal pressure of the pressure vessel reaches a predetermined standard.

[0033] Additionally, immersing the specimen in the above-mentioned gaseous ammonia environment may include loading the specimen, which has been bent using the above-mentioned FPBT, into a pressure vessel; performing argon purging to remove air inside the pressure vessel; purging the pressure vessel using a mixture of oxygen and argon in a predetermined ratio; and heating the internal temperature of the pressure vessel to a predetermined temperature.

[0034] At this time, the oxygen and argon mixed gas of the above-mentioned predetermined ratio can substantially correspond to a mixed gas of 0.9 vol.% O2 and 99.1 vol.% Ar.

[0035] In addition, the above-mentioned liquid ammonia may additionally include injecting water in a predetermined weight ratio.

[0036] In the above liquid ammonia method, the above predetermined temperature is -33.5 o It can substantially correspond to C, and when cooling the pressure vessel, the internal pressure of the pressure vessel can substantially correspond to 0 gauge pressure.

[0037] The above-mentioned predetermined standard can substantially correspond to 1.5 gauge pressure.

[0038] In addition, purging the pressure vessel using the above mixed gas may include purging so that the internal pressure of the pressure vessel corresponds to a 5 gauge pressure.

[0039] In addition, in the above gaseous ammonia method, the above predetermined temperature is 45 o It can effectively respond to C. Effects of the invention

[0040] According to the embodiments of the present invention as described above, an SCC evaluation can be performed using FPBT in a test device simulating an actual ammonia cargo tank environment, thereby providing a superior performance evaluation method that reflects the actual environment.

[0041] In addition, according to the embodiments, detailed specifications regarding how to simulate flow conditions inside the cargo tank, how to configure applied stress conditions, and how to set temperature conditions of the ammonia environment can be provided, thereby providing a method for evaluating the new IMO or equivalent regulations.

[0042] The effects obtainable from the present invention are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art from the description below. Brief explanation of the drawing

[0043] Figure 1 is a diagram illustrating a device used in the evaluation method for determining the suitability of metal materials in an ammonia environment by the IMO. Figures 2 and 3 are drawings illustrating the ammonia stress corrosion cracking mechanism. FIG. 4 is a flowchart illustrating an ammonia stress corrosion cracking evaluation method according to one embodiment of the present invention. FIG. 5 is a diagram illustrating a method for describing the flow conditions of a liquid cargo according to one embodiment of the present invention. FIG. 6 is a drawing for explaining stress conditions according to one embodiment of the present invention. FIGS. 7 and FIGS. 8 are drawings for explaining temperature conditions according to an embodiment of the present invention. FIG. 9 is a diagram illustrating a method for controlling dissolved oxygen concentration and moisture concentration in a liquid ammonia environment according to one embodiment of the present invention. FIG. 10 is a diagram illustrating a method for controlling oxygen concentration in a gaseous ammonia environment according to one embodiment of the present invention. Specific details for implementing the invention

[0044] Hereinafter, embodiments of the present invention are described in detail with reference to the attached drawings so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein. Furthermore, in order to clearly explain the present invention in the drawings, parts unrelated to the explanation have been omitted, and similar parts throughout the specification are denoted by similar reference numerals.

[0045] Throughout the specification, when a part is described as "including" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0047] Figures 2 and 3 are drawings illustrating the ammonia stress corrosion cracking mechanism.

[0048] Dissolved oxygen in ammonia can adsorb onto the surface of steel to form a film. At this time, a significant increase in corrosion potential may occur due to the oxygen film adsorbed on the surface. Figure 2 illustrates the potentiodynamic polarization curves of carbon steel in an LNH3 environment with or without ammonia.

[0049] However, during ship operation, localized areas where the oxygen film is not present may occur in the cargo hold due to stress and the flow of liquid cargo. This is illustrated in the form of a schematic diagram in Fig. 3.

[0050] SCC can occur as corrosion acts intensively due to galvanic corrosion in localized areas where an oxygen film is absent. In other words, it is necessary to consider conditions for simulating the internal environment of the cargo tank to evaluate SCC caused by ammonia.

[0052] FIG. 4 is a flowchart illustrating an ammonia stress corrosion cracking evaluation method according to one embodiment of the present invention.

[0053] First, a step (S410) of bending the specimen by applying a stress greater than a predetermined strength to the specimen using FPBT can be performed. The specimen may be prepared according to ISO 7539-2 and ISO 16540 standards, and it is preferable to maintain the specimen in a bent state under a stress greater than a predetermined strength using FPBT prior to immersion in an ammonia environment described later.

[0054] In test methods according to IMO regulations, the maximum stress applied to the specimen is limited to a level corresponding to the yield stress at room temperature, and a strain gauge may be attached to measure the stress. In the case of welded specimens, a strain gauge is attached to each side of the weld joint, and the specimen may be constrained to maintain its shape during the test.

[0055] However, as mentioned above, since SCC does not occur even when the IMO's above standards are exceeded, such standards lack technical basis. In one embodiment of the present invention, a stress condition (A) simulating a cargo hold environment is proposed, which will be described in detail later.

[0056] Subsequently, in one embodiment of the present invention, a step (S420) of immersing the above-described specimen for a predetermined period in a liquid or gaseous ammonia environment to which predetermined flow conditions are applied may be performed.

[0057] When using two specimens (one welded specimen and the other base material specimen), the IMO specifies that each be immersed in an ammonia environment for 30 days, and these environments may consist of liquid ammonia or gaseous ammonia.

[0058] However, in one embodiment of the present invention, considering the SCC generation mechanism described above in relation to FIGS. 2 and FIG. 3, it is proposed to perform ammonia immersion under a predetermined flow condition (B) that simulates the cargo tank environment.

[0059] After performing such ammonia environment immersion, an SCC inspection (S430) can be performed.

[0060] Specifically, all specimens may be inspected for SCC using an optical microscope with appropriate magnification. If cracks are present, their location and number must be clearly noted, and if necessary, the results may be verified through a dye penetrant test.

[0061] For specimens containing welds, it is desirable to specifically describe whether the location of the crack corresponds to the base material, the weld, or the heat-affected zone (HAZ).

[0062] If no cracks are observed on the surface, the specimen can be cut longitudinally at two different points, and a cross-sectional inspection can be performed on the corresponding cross-section at an appropriate magnification.

[0063] In addition, if pitting caused by corrosion is present, its presence and maximum depth can be reported.

[0065] FIG. 5 is a diagram illustrating a method for describing the flow conditions of a liquid cargo according to one embodiment of the present invention.

[0066] Flow conditions within the cargo tank are parameters involved in the adsorption and desorption of oxygen films, and as described above in relation to FIGS. 2 and FIGS. 3, they affect the occurrence of SCC. For example, if the oxygen film is desorbed due to a strong impact such as sloshing, it may affect the occurrence of SCC, whereas, conversely, an appropriate flow environment can accelerate the adsorption of the oxygen film and affect the prevention of SCC.

[0068] However, since the internal flow environment of the cargo tank changes depending on design conditions, it is difficult to present specific methods and figures. Furthermore, current IMO ammonia SCC test evaluation regulations do not consider the internal flow conditions of the cargo tank at all, which can be a significant disadvantage to evaluation reliability.

[0069] Accordingly, in one embodiment of the present invention, a predetermined flow condition (B) is proposed to be designed to correspond to the reciprocating motion condition of the experimental apparatus shown in FIG. 5, taking into account the flow condition inside the cargo tank.

[0070] FIG. 5 illustrates an example of utilizing a PSPC (Performance Standard for Protective Coatings) test apparatus. A PSPC test apparatus is equipment for testing and evaluating the corrosion resistance performance and quality of anti-corrosion coatings applied to the internal structures of a ship (e.g., ballast tanks, cargo holds, etc.). It is primarily equipment used to verify in a laboratory whether regulations established by the IMO (e.g., IMO Resolution MSC.215 (82)) are met, but in the present invention, a test apparatus for simulating a cargo hold environment does not need to be proposed.

[0071] When using such a PSPC test device, it is desirable that the reciprocating motion condition (B) requires at least one reciprocating motion every 3 seconds.

[0072] However, these flow conditions (B) may be defined differently depending on the design of the vessel, for example, depending on the type of vessel, the direction and frequency of reciprocating motion (left / right and / or up / down) may be defined differently.

[0074] FIG. 6 is a drawing for explaining stress conditions according to one embodiment of the present invention.

[0075] When stress exceeding the yield stress is applied to steel in an ammonia environment, a new slip plane without an oxygen film may be generated due to deformation. Figure 6 illustrates the propagation of SCC by this new plane in the form of a schematic diagram.

[0076] Newly formed surfaces are subject to locally concentrated corrosion, which can lead to SCC. Therefore, applied stress can be considered an important parameter in ammonia SCC testing.

[0077] Current IMO ammonia SCC test evaluations apply yield stress as described above, but one embodiment of the present invention proposes applying stress exceeding this when considering the SCC mechanism described above.

[0078] Accordingly, in one embodiment of the present invention, a stress greater than a predetermined strength corresponding to the stress condition (A) of FIG. 4 is proposed to correspond to a stress greater than the yield stress at room temperature, which can be distinguished from the current IMO method.

[0079] Preferably, in one embodiment of the present invention, the stress greater than or equal to the predetermined strength is proposed to correspond to a stress within the range greater than or equal to the yield stress and less than or equal to the tensile stress.

[0081] FIGS. 7 and FIGS. 8 are drawings for explaining temperature conditions according to an embodiment of the present invention.

[0082] Temperature (C) is a parameter involved in corrosion rate and ammonia condensation, and it affects the occurrence of SCC.

[0083] During the operation of an ammonia carrier, the boiling point at atmospheric pressure (-33.5 o C) Although managed below, during cargo loading and unloading, as shown in Fig. 7, up to 45 oThe temperature can rise to C. At this time, ammonia may re-condense in the cargo tank at a lower temperature, causing SCC. Figure 8 shows whether SCC occurs due to the temperature difference between the specimen and gaseous ammonia.

[0084] Currently, under the IMO ammonia SCC test evaluation regulations, -33.5, -20 o The ammonia temperature is set to C, Ambient, and three other values. However, in a preferred embodiment of the present invention, it is proposed to set the temperature considering the actual operating conditions shown in FIG. 7, and accordingly, the ammonia environment is -33.5 o C or higher 45 o It is proposed that it be set to have temperature conditions of C or lower.

[0086] FIG. 9 is a diagram illustrating a method for controlling dissolved oxygen concentration and moisture concentration in a liquid ammonia environment according to one embodiment of the present invention.

[0087] According to the present embodiment, an FPBT specimen can first be loaded into a pressure vessel (S910). Then, it is preferable to create a vacuum after argon purging to remove air from inside the pressure vessel (S920).

[0088] After that, the pressure vessel can be purged (S930) using a mixture of oxygen and argon in a predetermined ratio. At this time, it is preferable to purge the pressure vessel using a mixture of 0.9 vol.% O2 and 99.1 vol.% Ar gas as specified in IGC Code 17.12.8, and the pressure can reach ~1 gauge pressure (barg).

[0089] After that, the outlet valve of the pressure vessel can be opened, and liquid ammonia can be charged (S940) so that the specimen is submerged in the liquid ammonia. At this time, in the case of additional moisture conditions, water (H2O) equivalent to 0.1 weight % may be injected through a pump. At this time, it is preferable to charge enough liquid ammonia (LNH3) so that the FPBT specimen is sufficiently submerged, and in the actual example, 1.4 kg of LNH3 was charged, but it is not limited to this.

[0090] After that, the internal temperature of the pressure vessel can be cooled to a predetermined temperature (S950). In one embodiment, the internal temperature of the pressure vessel is -33.5 o It is suggested to cool to C, at which time the internal pressure can correspond to ~0 gauge pressure (barg).

[0091] After that, the oxygen and argon mixed gas of the above-mentioned ratio can be charged (S960) so that the internal pressure of the pressure vessel becomes a predetermined standard. That is, a mixed gas of 0.9 volume % O2 + 99.1 volume % Ar can be charged so that the internal pressure of the pressure vessel becomes 1.5 gauge pressure.

[0092] This embodiment can be viewed as a method utilizing Henry's Law, through which the dissolved oxygen concentration and moisture concentration in a liquid ammonia environment can be efficiently controlled.

[0094] FIG. 10 is a diagram illustrating a method for controlling oxygen concentration in a gaseous ammonia environment according to one embodiment of the present invention.

[0095] According to the present embodiment, first, a specimen bent using FPBT is loaded into a pressure vessel (S1010), and argon purging (Ar purging) is performed (S1020) to remove air inside the pressure vessel. It is desirable to secure vacuum conditions through this.

[0096] After that, the pressure vessel can be purged (S1030) using a mixture of oxygen and argon in a predetermined ratio. It is preferable to sufficiently purge the pressure vessel using a mixture of 0.9 vol.% O2 and 99.1 vol.% Ar gas, as in the embodiment of FIG. 9 described above, at which time the pressure can reach ~5 gauge pressure (barg).

[0097] After that, the internal temperature of the pressure vessel can be heated to a predetermined temperature (S1040). Preferably, the internal temperature of the pressure vessel is 45 o It can be heated up to C.

[0098] Using this method, oxygen concentration can be efficiently controlled in a gaseous ammonia environment.

[0100] The detailed description of the preferred embodiments of the present invention disclosed above is provided to enable those skilled in the art to implement and practice the present invention. Although the present invention has been described with reference to preferred embodiments, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the scope of the invention. For example, those skilled in the art may utilize each configuration described in the above embodiments in a manner that combines with one another.

[0101] Accordingly, the present invention is not intended to be limited to the embodiments shown herein, but to be given the broadest scope consistent with the principles and novel features disclosed herein. Explanation of the symbols

[0102] 110: Vendor 120: Jig A: Stress conditions B: Flow conditions C: Temperature conditions

Claims

Claim 1 A method for evaluating ammonia stress corrosion cracking (SCC), comprising: bending a specimen by applying a stress greater than a predetermined strength to the specimen using a Four Point Bending Test (FPBT); immersing the specimen for a predetermined period in a liquid or gaseous ammonia environment with predetermined flow conditions; and inspecting the stress corrosion cracking (SCC) of the specimen using an optical microscope. Claim 2 A method for evaluating ammonia stress corrosion cracking, wherein the predetermined flow conditions in claim 1 correspond to the reciprocating motion conditions of an experimental device considering the flow conditions inside a cargo tank. Claim 3 In claim 2, the reciprocating motion condition requires at least one reciprocating motion every 3 seconds, an ammonia stress corrosion cracking evaluation method. Claim 4 A method for evaluating ammonia stress corrosion cracking, wherein, in claim 1, the stress greater than the predetermined strength corresponds to a stress greater than the yield stress at room temperature. Claim 5 A method for evaluating ammonia stress corrosion cracking, wherein, in claim 1, the stress greater than or equal to the predetermined strength corresponds to a stress within the range greater than or equal to yield stress and less than or equal to tensile stress. Claim 6 In claim 1, the ammonia environment is -33.5 o C or higher 45 o A method for evaluating ammonia stress corrosion cracking, set to have temperature conditions of C or lower. Claim 7 A method for evaluating ammonia stress corrosion cracking according to claim 1, wherein the specimen comprises a base material and a weldment specimen, and the predetermined period of immersion in the ammonia environment corresponds to a period of 30 days or more. Claim 8 A method for evaluating ammonia stress corrosion cracking according to claim 1, wherein immersing the specimen in the liquid ammonia environment comprises: loading the specimen, which is bent using the FPBT, into a pressure vessel; performing argon purging to remove air inside the pressure vessel; purging the pressure vessel using a mixed gas of oxygen and argon in a predetermined ratio; opening the discharge valve of the pressure vessel and loading the liquid ammonia so that the specimen is submerged in the liquid ammonia; cooling the internal temperature of the pressure vessel to a predetermined temperature; and loading the mixed gas of oxygen and argon in a predetermined ratio so that the internal pressure of the pressure vessel becomes a predetermined standard. Claim 9 A method for evaluating ammonia stress corrosion cracking according to claim 1, wherein immersing the specimen in the gaseous ammonia environment comprises: loading the specimen bent using the FPBT into a pressure vessel; performing argon purging to remove air inside the pressure vessel; purging the pressure vessel using a mixed gas of oxygen and argon in a predetermined ratio; and heating the internal temperature of the pressure vessel to a predetermined temperature. Claim 10 A method for evaluating ammonia stress corrosion cracking, wherein, in claim 8 or 9, the oxygen and argon mixed gas of the predetermined ratio corresponds substantially to a mixed gas of 0.9 vol.% O2 and 99.1 vol.% Ar. Claim 11 A method for evaluating ammonia stress corrosion cracking, wherein, in claim 8, the charging of the liquid ammonia further comprises injecting water in a predetermined weight ratio. Claim 12 In claim 8, the above-mentioned predetermined temperature is -33.5 o A method for evaluating ammonia stress corrosion cracking, substantially corresponding to C, wherein the internal pressure of the pressure vessel corresponds substantially to 0 gauge pressure when the pressure vessel is cooled. Claim 13 In claim 8, the above-mentioned predetermined standard is an ammonia stress corrosion cracking evaluation method substantially corresponding to 1.5 gauge pressure. Claim 14 A method for evaluating ammonia stress corrosion cracking according to claim 9, wherein purging the pressure vessel using the mixed gas comprises purging so that the internal pressure of the pressure vessel corresponds to a 5 gauge pressure. Claim 15 In claim 9, the predetermined temperature is 45 o A method for evaluating ammonia stress corrosion cracking that substantially corresponds to C.

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