Nitric Acid-Free Pickling for Low-Chrome Ferrite Steel
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Solution Overview
Problem
Current pickling processes for low-chrome ferrite-based cold-rolled stainless steel sheets are inefficient and costly due to the use of nitric acid, leading to NOx emissions and high production costs, and lack a suitable method for high-speed production with high-quality surface finishes.
Innovation Solution
A pickling process using a mixed acid solution without nitric acid, comprising sulfuric acid, hydrofluoric acid, and hydrogen peroxide, with controlled concentrations and potentials, combined with neutral salt and sulfuric acid electrolyte treatments, to rapidly remove oxide scales and achieve high-quality surfaces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If nitric acid is used as pickling solution, then oxide scale is effectively removed from steel sheet surface, but NOx emissions and nitric nitrogen in waste acids increase
Solution Approach 1:
The patent changes the chemical composition parameters of the pickling solution by replacing nitric acid with a nitric acid-free formulation containing sulfuric acid, hydrofluoric acid, and hydrogen peroxide. This parameter substitution eliminates NOx emissions while maintaining effective oxide scale removal capability through alternative chemical mechanisms.
Solution Approach 2:
The patent converts the harmful effect of nitric acid (NOx emissions) into a beneficial process by using hydrogen peroxide as an oxidizing agent that decomposes into water and oxygen, eliminating harmful emissions while maintaining the necessary oxidation function for pickling.
2Manufacturing precision
If nitric acid is used in pickling process, then oxide scale removal is effective, but production costs increase due to pollution prevention equipment
Solution Approach 1:
The patent extracts and removes nitric acid from the pickling solution composition, eliminating the need for expensive pollution prevention equipment while maintaining effective oxide scale removal through alternative chemicals that do not require such equipment.
Solution Approach 2:
The patent replaces expensive nitric acid and associated pollution control infrastructure with cheaper alternative chemicals (sulfuric acid, hydrofluoric acid, hydrogen peroxide) that can be easily disposed of or degraded, reducing both material and equipment costs.
3Adaptability or versatility
If low-chrome ferrite cold-rolled stainless steel sheets are used, then corrosion resistance is reduced, but higher amounts of steel dissolve in acid solution increasing NOx generation
Solution Approach 1:
The patent changes the oxidizing agent from nitric acid to hydrogen peroxide, which provides sufficient oxidation capability to handle low-chrome ferrite steel with lower corrosion resistance without generating NOx emissions, thus adapting the pickling process to materials with varying corrosion resistance levels.
4Productivity
If conventional pickling methods are used for high-speed production, then productivity is limited, but surface quality requirements cannot be met
Solution Approach 1:
The patent optimizes the chemical parameters of the pickling solution (concentrations of sulfuric acid, hydrofluoric acid, and hydrogen peroxide) and controls process parameters (temperature, time, potential) to achieve both high pickling speed and high surface quality, resolving the trade-off between productivity and precision.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This process eliminates NOx emissions, reduces production costs, and enables high-speed pickling with improved surface quality and productivity, effectively removing Si oxide and other oxides within 10-100 seconds, while maintaining surface gloss and corrosion resistance.
Implementation Method 1
Nitric acid reduces the pH level of a pickling bath to increase the activity of hydrofluoric acid, and oxidizes Fe2+ ions into Fe3+ ions to maintain the oxidation-reduction potential at a level suitable for pickling.
Implementation Method 2
oxide scale is removed by various methods such as: physical descaling including brushing or shot ball blasting; electrolytic descaling using a sodium sulfate electrolyte, a sulfuric acid electrolyte, or a nitric acid electrolyte; and chemical descaling using a salt bath or an acid mixture.
Implementation Method 3
electrolytic descaling using a sodium sulfate electrolyte, a sulfuric acid electrolyte, or a nitric acid electrolyte
Data Source
Figure 1A~1B
Figure 2A~2B
Figure 3A~3B
AI summary
The present invention relates to a method for pickling the surface of a steel sheet at high speed during the manufacturing of a ferrite-based cold-rolled stainless steel sheet that requires good surface qualities. The method for pickling a low-chrome ferrite-based cold-rolled stainless steel sheet at high speed involves pickling a ferrite-based cold-rolled stainless steel sheet which contains 14% or less of chrome and which has undergone a degreasing and annealing process, wherein the method comprises a neutral salt electrolyte treatment including the electrolytic removal of chrome-rich scale from the surface of the steel sheet using an electrolytic solution in which sodium sulfate is used as an electrolyte; a sulfuric acid electrolyte treatment including the electrolytic removal of iron-rich scale using an electrolytic solution in which sulfuric acid is used as an electrolyte; and a mixed acid treatment including immersing the steel sheet in a mixed acid solution containing sulfuric acid, hydrofluoric acid, and hydrogen peroxide, wherein the whole pickling process is completed within 15 to 240 seconds. According to the present invention, the use of nitric acid in pickling a ferrite-based cold-rolled stainless steel sheet can be avoided, thus reducing the burden of installing an NOx removal facility and a denitrification facility, and furthermore, pickling may be adjusted in terms of the hydrogen peroxide concentration and hydrofluoric acid concentration, thus making the process easy to control and suitable for high-speed production. The quality of the steel sheet having undergone the pickling process of the present invention is improved as compared to that of the steel sheet of existing pickling process, the result being that a high-quality ferrite-based cold-rolled stainless steel sheet may be produced.