High-Purity Ferritic Stainless Steel Sn Coating Corrosion
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Solution Overview
Problem
High purity ferritic stainless steel faces limitations in corrosion resistance while maintaining formability and low material costs, as existing methods either increase costs or compromise formability and manufacturability by adding rare elements.
Innovation Solution
The development of a high purity ferritic stainless steel with a modified coating that satisfies specific X-ray intensity ratios and includes elements like Sn, Cr, and Ti, combined with annealing and pickling processes, enhances corrosion resistance without relying on rare elements, achieving a pitting potential comparable to or exceeding SUS304.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Cr and Mo are added to improve corrosion resistance, then corrosion resistance is improved, but material cost increases and formability deteriorates
Solution Approach 1:
The invention changes the chemical composition parameters by adding Sn (0.01-1.0 mass%) instead of relying on high Cr and Mo content. This parameter substitution allows achieving corrosion resistance comparable to SUS304 while maintaining the low Cr (12-17 mass%) formulation that preserves formability and economy.
Solution Approach 2:
The invention replaces expensive alloying elements (Cr, Mo) with cheaper Sn addition combined with surface coating modification. This approach achieves the desired corrosion resistance at lower material cost without compromising formability, effectively substituting expensive materials with more economical alternatives.
2Reliability
If bright annealing is used to modify the coating, then corrosion resistance is improved and cost is controlled, but new surfaces exposed by forming and polishing lack corrosion resistance
Solution Approach 1:
The invention performs preliminary action by adding Sn to the steel composition before manufacturing processes. This ensures that Sn is present in the material from the beginning, so when new surfaces are exposed during forming and polishing, they inherently contain Sn and can develop protective coatings, eliminating the need for post-forming corrosion protection treatments.
Solution Approach 2:
The invention applies local quality by concentrating Sn at the steel surface through selective addition and processing. The surface layer develops a specific composition with Sn enrichment (achieving Sn/Fe atomic ratio of 0.01-0.1), providing enhanced corrosion resistance specifically at the surface where it is most needed, while the bulk material maintains its formability characteristics.
3Reliability
If P is added to improve weather and corrosion resistance, then corrosion resistance is improved, but manufacturability, formability, and weldability are inhibited
Solution Approach 1:
The invention substitutes P (which causes manufacturing issues) with Sn combined with bright annealing treatment. This replacement achieves similar or superior corrosion resistance without the detrimental effects on manufacturability, formability, and weldability that P introduces, effectively using a more versatile element in combination with surface treatment.
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 approach results in an alloy-saving type of high purity ferritic stainless steel with corrosion resistance equivalent to or better than SUS304, maintaining formability and cost-effectiveness, as demonstrated by improved pitting potential and cycle test results.
Implementation Method 1
annealing at a temperature of 850 to 1000°C
Implementation Method 2
pickling in an aqueous solution containing 5 mass% or more of nitric acid
Data Source
Figure 1
AI summary
The present invention provides an alloy-saving type of high purity ferritic stainless steel excellent in corrosion resistance and a method of production of the same. A high purity ferritic stainless steel excellent in corrosion resistance comprised of, by mass%, C: 0.001 to 0.02%, Si: 0.01 to 0.6%, Mn: 0.01 to 0.6%, P: 0.005 to 0.04%, S: 0.0001 to 0.01%, Cr: 13 to 22%, N: 0.001 to 0.02%, Al: 0.005 to 0.05%, Sn: 0.001 to 1%, and a balance of Fe and unavoidable impurities, said steel characterized by satisfying the two relations of the following formula (1) and formula (2) where I(Fe), I(Cr), I(Sn), and I(O) are the X-ray intensities of the Fe oxides, Cr oxides, Sn oxides, and other detected oxides at the steel surface measured by an X-ray photoelectron spectrometer: 0<IFe/ICr<5 and 0<IO/ISn<3 To improve the effect of modification of the coating by addition of Sn, the method comprises annealing at a temperature higher than 800°C for final annealing, then cooling this by a cooling rate of 10°C/sec or more down to 700°C or less, holding this in a 200 to 700°C temperature range for at least 1 minute for cooling, then pickling in an aqueous solution containing, by wt%, at least 5% nitric acid or performing bright annealing while making a dew point of the atmospheric gas -50°C to -20°C.