Floating iron powder or embedded iron:
Free iron can rust and corrode stainless steel on any surface. Therefore, it must be cleared. Floating powder can generally be removed along with the dust. Some are very sticky and must be treated as embedded iron. In addition to dust, there are many sources of surface iron, which include cleaning with plain carbon steel wire brushes and shot peening with sand, glass beads or other abrasives previously used on plain carbon steel, low alloy steel or cast iron, or Regrind the aforementioned non-stainless steel products near stainless steel components and equipment.
If the stainless steel is not protected during the cutting or lifting process, the steel wire rope, spreader and iron on the work surface can easily embed or stain the surface. Ordering requirements and post-fabrication inspections can prevent and detect the presence of free iron. ASTM Standard A380 [3] specifies a rust test method for inspecting iron or steel particles on stainless steel surfaces. This test should be used when absolutely no iron is required.
If the results are satisfactory, wash the surface with clean pure water or nitric acid until the dark blue color has completely disappeared. As noted in Standard A380 [3], if the rust test solution cannot be completely removed, this test method is not recommended on technical surfaces of equipment, ie direct contact surfaces used in the production of human consumer products. A simpler test method is to expose it to water for 12 to 24 hours to check for rust spots. This test is insensitive and time-consuming. These are testing tests, not cleaning methods. If iron is found, it must be cleaned up by chemical and electrochemical methods described later.
Scratches:
Scratches and other rough surfaces must be mechanically cleaned to prevent the build-up of process lubricants or products and/or dirt, typically with a stainless steel special polisher. If the stainless steel is heated to a certain high temperature in the air during welding or grinding, the chromium oxide thermal tempering color will appear on both sides of the weld, the lower surface and the bottom of the weld. The hot tempered color is thinner than the oxide protective film and is clearly visible. Color depends on thickness and can be found in iridescent, blue, violet to yellowish and brown. Thicker oxides are generally black. It is caused by staying at high temperatures or prolonged periods of time at higher temperatures. When any of these oxide layers are present, the chromium content of the metal surface is reduced, resulting in reduced corrosion resistance in these areas. In this case, not only should the hot tempering color and other oxide layers be eliminated, but the chromium-depleted metal layer beneath them should also be cleaned.
Rust spots:
Rust is sometimes seen on stainless steel products or equipment before or during production, indicating a heavily contaminated surface. Rust must be removed before equipment is put into service, and thoroughly cleaned surfaces should be inspected by iron and/or water tests.
Rough grinding and machining:
Both grinding and machining result in a rough surface with defects such as grooves, overlaps and burrs. Each defect may also damage the metal surface to such a depth that the damaged metal surface cannot be removed by methods such as pickling, electropolishing, or shot peening (eg, dry blasting, glass beads for abrasives). Rough surfaces can be the source of corrosion and deposition products, and cleaning of weld defects or excess weld reinforcement before re-welding cannot be ground by rough grinding. In the latter case, it should be ground with fine abrasives.
Welding arc marks:
When the welder strikes the arc on the 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 bead that has been welded or on the side of the weld joint. The arc strike is then fused into the weld.
Welding spatter:
Welding spatter has a lot to do with the welding process. For example: GTAM (Gas Tungsten Arc Welding) or TIG (Inert Gas Tungsten Arc Welding) no spatter. However, GMAW (Gas Metal Arc Welding) and FCAW (Flux Core Arc Welding) welding processes can cause a lot of spatter if the welding parameters are not used properly. When this happens, the parameters must be adjusted. If you want to solve the problem of welding spatter, you should apply a spatter inhibitor on each side of the joint before welding, which can eliminate the adhesion of spatter. After welding, this anti-splash agent and various spatters can be easily removed without damaging the surface or causing minor damage.
Welding processes using flux include manual welding, flux-cored arc welding and submerged arc welding. These welding processes leave small flux particles on the surface that cannot be removed by ordinary cleaning methods. This particle will be a source of crevice corrosion, and mechanical cleaning methods must be used to remove these residual flux.
Welding Defects:
Welding defects such as undercuts, lack of penetration, dense pores and cracks not only reduce the firmness of the joint, but also become a source of crevice corrosion. They also entrain solid particles when cleaning operations are performed to improve this result. These defects can be repaired by re-welding or re-grinding and re-welding.
Oils and Greases:
Organic substances such as oil, grease and even fingerprints can be a source of corrosion for localized corrosion. Because these substances act as barriers, they interfere with chemical and electrochemical cleaning and must be removed thoroughly. ASTM A380 has a simple water break (WATERBREAK) test to detect organic contaminants. During the test, water was poured from the top of the vertical surface, and the water was divided along the perimeter of the organic material during the downward flow. Flux and/or acidic chemical cleaners can remove oil and grease.
Residual adhesive:
When the tape and protective paper are removed, there is always a portion of the adhesive left on the stainless steel surface. If the adhesive is not yet hard, it can be removed with an organic flux. However, when exposed to light or air, the adhesive hardens, forming a source of crevice corrosion. Mechanical cleaning with fine abrasives is then required.
Paint pen print:
The effects of these contaminants are similar to those of oils and greases. Washing with a clean brush and clean water or alkaline cleaner is recommended, and high pressure water or steam can also be used. Stainless steel mainly composed of ferrite in the use state. 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 in the manufacture of atmospheric resistance. , Parts corroded by water vapor, water and oxidizing acid. This type of steel has disadvantages such as poor plasticity, significantly reduced post-weld plasticity and corrosion resistance, which limits its application. The application of out-of-furnace refining technology (AOD or VOD) can greatly reduce interstitial elements such as carbon and nitrogen, thus making this type of steel widely used.
