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What to Do if the Stainless Steel Surface is Damaged?

Dust

 Manufacturing is often carried out in dusty places, 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, dust with adhesion requires high-pressure water or steam to clean.

What to Do if the Stainless Steel Surface is Damaged? 1

Floating iron powder or embedded iron

 Free iron on any surface will rust and corrode stainless steel. Therefore, it must be removed. Floating powder can generally be removed with 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 peening with sand, glass beads or other abrasives previously used on plain carbon steel, low alloy steel or cast iron parts, or grinding the non-stainless steel products mentioned above near stainless steel parts and equipment. If stainless steel is not protected during unloading or hoisting, iron from wire ropes, hoists and work surfaces can easily become embedded in or stain the surface.

 Order requirements and post-production inspections can prevent and detect the presence of free iron. ASTM standard A380[3] specifies a rust test method for checking the surface of stainless steel for iron or steel particles. This test method should be used when it is required that must be  absolutely no iron present. If the results are satisfactory, the surface should be washed with clean pure water or nitric acid until the dark blue color disappears completely.

 As indicated in standard A380[3], if the rust test solution cannot completely remove it, this test method is not recommended for process surfaces of equipment, that is, surfaces that are in direct contact with the production of human consumption products. A simpler test method is to expose it in water for 12 to 24 hours and check for rust spots. This test has poor sensitivity and is time-consuming. These are detection tests, not cleaning methods. If iron is found, it must be cleaned using the chemical and electrochemical methods described below.

What to Do if the Stainless Steel Surface is Damaged? 2

Scratches

 To prevent the accumulation of process lubricants or products and/or dirt, scratches and other rough surfaces must be mechanically cleaned, usually with a stainless steel polisher. Heat tempering color and other oxide layers If the stainless steel is heated to a certain high temperature in air during welding or grinding, chromium oxide heat tempering color will appear on both sides of the weld, the lower surface and the bottom of the weld. Heat tempering color is thinner than the oxide protective film and is clearly visible. The color depends on the thickness and can be rainbow, blue, purple to light yellow and brown. Thicker oxides are generally black. It is caused by high temperature or long-term stay at a higher altitude. When any of these oxide layers appear, the chromium content of the metal surface will be reduced, resulting in reduced corrosion resistance in these areas. In this case, not only should the heat tempering color and other oxide layers be eliminated, but also the chromium-poor metal layer below them should be cleaned.

 Rust

 Rust is sometimes seen on stainless steel products or equipment before or during production, indicating that the surface is seriously contaminated. Before the equipment is put into use, the rust must be removed and the thoroughly cleaned surface should be inspected by iron test and/or water test.

 Rough grinding and machining

 Grinding and machining will cause surface roughness, leaving 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 peening (such as dry sandblasting, abrasive glass beads). Rough surface can become the birthplace of corrosion and deposition products. Cleaning weld defects before re-welding or removing excess weld reinforcement height cannot be ground by coarse grinding. In the latter case, fine abrasives should be used for grinding.

What to Do if the Stainless Steel Surface is Damaged? 3

Welding spark 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 welded weld or on the side of the weld joint. Then melt the arc mark 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) does not produce spatter. However, GMAW (gas metal arc welding) and FCAW (flux cored arc welding) processes can produce 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, apply a spatter repellent on each side of the joint before welding, which will eliminate the adhesion of the spatter. This spatter repellent and various spatters can be easily cleaned off after welding, with no damage to the surface or slight damage.

 Flux

 The processes that use flux for welding are manual welding, flux cored arc welding and submerged arc welding. These welding processes will leave fine flux particles on the surface, which cannot be removed by ordinary cleaning methods. These particles will be the source of corrosion for 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 strength of the joint, but also become a source of corrosion for crevice corrosion. They can also carry solid particles during cleaning operations to improve this result. These defects can be repaired by re-welding or re-welding after grinding.

 Oil and grease

 Organic substances such as oil, grease and even fingerprints can become a source of corrosion for localized corrosion. Because these substances can act as a barrier, they will affect the chemical and electrochemical cleaning effect and must be thoroughly cleaned. ASTM A380 has a simple water break (WATERBREAK) test to detect organic contaminants. During the test, water is poured from the top of the vertical surface, and the water will separate around the organic material as it flows downward. Flux and/or acid chemical cleaning agents can remove oil and grease.

What to Do if the Stainless Steel Surface is Damaged? 4

Residual adhesive

 When removing tape and protective paper, some adhesive will always remain on the stainless steel surface. If the adhesive has not hardened, it can be removed with an organic flux. However, when exposed to light and/or air, the adhesive hardens and forms a source of corrosion for crevice corrosion. Mechanical cleaning with a fine abrasive is then required.

 Paint, chalk and marker marks

 The effects of these contaminants are similar to those of oil and grease. Washing with a clean brush and clean water or an alkaline cleaner is recommended, and high pressure water or steam washing can also be used.

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