Corrosion resistance and types of stainless steel

1. Dust
Production is often carried out in dusty areas, and there is often a lot of dust in the air, which constantly falls on the surface of the equipment. They can be removed with water or alkaline solutions. However, adherent dirt requires high-pressure water or steam to clean.
2. Floating iron powder or embedded iron
On any surface, free iron will rust and cause corrosion in stainless steel. Therefore, it must be cleared. Floating powder can generally be removed along with the dust. Some have strong adhesion and must be treated as embedded iron. In addition to dust, there are many sources of surface iron, including cleaning with ordinary carbon steel wire brushes and shot blasting with sand, glass beads or other abrasives previously used on ordinary carbon steel, low alloy steel or iron castings, or Grind the previously mentioned non-stainless steel products near stainless steel parts and equipment. If stainless steel is not protected during the unloading or lifting process, the iron on the wire rope, spreader, and work surface can easily become embedded or stain the surface.
The corrosion resistance of stainless steel and its type ordering requirements and post-production inspection can prevent and detect the presence of free iron. ASTM standard A380 specifies the rust test method for checking iron or steel particles on the surface of stainless steel. This test method should be used when the absolute absence of iron is required. If the result is satisfactory, the surface should be washed with clean pure water or nitric acid until the dark blue color completely disappears.
As stated in Standard A380, if the rust test solution cannot be completely removed, this test method is not recommended for use on process surfaces of equipment, i.e. direct contact surfaces used to produce products for human consumption. A relatively simple test method is to expose it to water for 12 to 24 hours and check whether there are rust spots. This test is insensitive and time-consuming. These are detection tests, not cleaning methods. If iron is found, it must be cleaned using the chemical and electrochemical methods described later.
3. Scratches
To prevent the accumulation of process lubricants or products and/or contaminants, scratches, and other rough surfaces must be mechanically cleaned.
4. Thermal tempering color and other oxide layers
If stainless steel is heated to a certain high temperature in the air during welding or grinding, chromium oxide thermal tempering color will appear on both sides of the weld, the lower surface, and the bottom of the weld. The thermal tempering color is thinner than the oxidation protective film and is visible. The color depends on the thickness and can range from iridescent, blue, and purple to light yellow and brown. Thicker oxides are generally black. It is caused by exposure to high temperatures or staying at higher temperatures for a long time. When any of these oxide layers appear, the chromium content of the metal surface is reduced, causing these areas to be less corrosion-resistant. In this case, not only should the thermal temper color and other oxide layers be eliminated, but the chromium-poor metal layer beneath them should also be cleaned.
5. Rust spots
Sometimes you will see rust on stainless steel products or equipment before or during the production process, which indicates that the surface is seriously contaminated. Rust must be removed before equipment is put into service, and thoroughly cleaned surfaces should be inspected by an iron test and/or a water test.
6. Rough grinding and machining
Both grinding and machining can result in rough surfaces with defects such as grooves, overlaps, and burrs. Each defect may also damage the metal surface to a certain depth so that the damaged metal surface cannot be cleaned by pickling, electropolishing, or shot blasting. Rough surfaces can become the source of corrosion and deposition products. Coarse grinding cannot be used to clean weld defects or remove excess weld reinforcement before re-welding. In the latter case, fine abrasives should be used again.
7. Welding arc marks
When a welder strikes an arc on a metal surface, it will cause surface roughness defects. The protective film is damaged, leaving a potential source of corrosion. The welder should strike the arc on the already welded bead or the side of the weld joint. The pilot arc trace is then melted into the weld.
8. Welding spatter
Welding spatter has a lot to do with the welding process. For example: GTAM (gas tungsten arc welding) or TIG (tungsten inert gas welding) without spatter. However, when using the two welding processes of GMAW (gas-shielded metal arc welding) and FCAW (flux-cored arc welding), improper use of welding parameters will cause a large amount of spatter. When this occurs, parameters must be adjusted. If you want to solve the problem of welding the spatter, you should apply an anti-spatter agent on each side of the joint before welding, which can eliminate the adhesion of the spatter. After welding, this anti-spatter agent and various spatters can be easily cleaned away without damaging the surface or causing minor damage.
9. Flux
Welding processes using flux include manual welding, flux-cored arc welding, and submerged arc welding. These welding processes will leave small flux particles on the surface, which cannot be removed by ordinary cleaning methods. These particles will be the source of crevice corrosion, and mechanical cleaning methods must be used to remove these residual fluxes.
10. Welding defects
Welding defects such as undercuts, incomplete welding, dense pores, and cracks not only reduce the strength of the joint but also become sources of crevice corrosion. When cleaning operations are performed to improve this outcome, they can also entrain solid particles. These defects can be repaired by re-welding or grinding and re-welding. 11. Oil and grease
Organic substances such as oil, grease, and even fingerprints can become sources of localized corrosion. Because these substances act as a barrier, they interfere with chemical and electrochemical cleaning and must be completely removed. ASTM A380 has a simple water break (WATERBREAK) test to detect organic pollutants. In the experiment, water is poured from the top of a vertical surface and as it flows downward it separates around the organic material. Flux and/or acidic chemical cleaners remove oil and grease.
12. Residual adhesive
When the tape and protective paper are removed, some adhesive will always remain on the stainless steel surface. If the adhesive is not hard yet, it can be removed with an organic solvent. However, when exposed to light and/or air, the adhesive hardens, forming a source of crevice corrosion. Mechanical cleaning with fine abrasives is then required. 13. Paint, chalk and markers
The effects of these contaminants are similar to those of oil and grease. It is recommended to wash with a clean brush and clean water or an alkaline cleaning agent. You can also use high-pressure water or steam cleaning.
Stainless steel with a mainly ferrite structure under use. The chromium content is between 11% and 30%, and it has a body-centered cubic crystal structure. This type of steel generally does not contain nickel and sometimes contains a small amount of Mo, Ti, Nb, and other elements. This type of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, and excellent stress corrosion resistance. It is mostly used to make atmospheric-resistant steel. , parts corroded by steam, water, and oxidizing acids. This type of steel has shortcomings such as poor plasticity, significantly reduced plasticity, and corrosion resistance after welding, which limits its application. The application of outside-furnace refining technology (AOD or VOD) can greatly reduce interstitial elements such as carbon and nitrogen, thus making this type of steel widely used.