What to do if the surface of stainless steel is damaged? For most people, this should be a big problem! So today, the editor will lead you to solve this headache. You will not be afraid of stainless steel damage in the future!

First of all, let’s take a look at what causes damage to the surface of stainless steel; there are many reasons for damage to the surface of stainless steel, including dust, floating iron powder or embedded iron, scratches, rust spots, rough grinding and machining, welding arc marks, Welding spatter, flux, welding defects, oil and grease, residual adhesive paint, chalk and marking pen marks, etc. The different solutions for each cause are also different!


dust

Production is often carried out in dusty venues. There are often many dusts in the air, which constantly fall on the surface of the equipment. They can be removed with water or alkaline solution. However, adhesion of dirt requires high-pressure water or steam to clean up.

Floating iron powder or embedded iron

On any surface, free iron will rust and cause corrosion of stainless steel. Therefore, it must be cleared. Floating powder can generally be removed with the dust. Some have strong adhesion and must be treated with embedded iron. In addition to dust, there are many sources of iron on the surface, including cleaning with ordinary carbon steel wire brushes and shot peening with sand, glass beads or other abrasives that have been used on carbon steel, low alloy steel or cast iron parts, or Grind the aforementioned non-stainless steel products near stainless steel parts and equipment. In the process of unloading or hoisting, if no protective measures are taken for the stainless steel, the steel wire rope, spreader and the iron on the work surface are easy to embed or stain the surface.

Ordering requirements and inspection after production can prevent and find the existence of free iron. ASTM Standard A380 [3] specifies the rust test method for inspecting iron or steel particles on the surface of stainless steel. This test method should be used when it is required that iron must not be present. If the result is satisfactory, use clean pure water or nitric acid to wash the surface until the dark blue color disappears completely.

As the standard A380 [3] pointed out, if the rust test solution cannot be completely removed, it is not recommended to use this test method on the process surface of the equipment, that is, the direct contact surface used to produce human consumer products. A relatively simple test method is to expose for 12 to 24 hours in water to check for rust spots. This test is poorly sensitive and time-consuming. These are inspection tests, not cleanup methods. If iron is found, it must be cleaned up by the chemical and electrochemical methods described later.

Scratches

In order to prevent process lubricants or products and dirt from accumulating, scratches and other rough surfaces must be mechanically cleaned. Generally, they are removed with a special stainless steel polishing machine. Thermal tempering color and other oxide layers if during welding or grinding Medium stainless steel is heated to a certain high temperature in the air, and chromium oxide thermal tempering color will appear on both sides of the weld, the lower surface and the bottom of the weld. The heat-tempered color is thinner than the oxide protective film and is clearly visible. The color is determined by the thickness, which can be iridescent, blue, purple to pale yellow and brown. Thicker oxides are generally black. It is caused by staying at high temperature or high temperature for a long time. When any of these oxide layers appear, the chromium content on the metal surface will decrease, causing the corrosion resistance of these areas to decrease. In this case, not only should the thermal tempering color and other oxide layers be eliminated, but also the chromium-poor metal layer under them should be cleaned.

Rust spots

Rust on stainless steel products or equipment is sometimes seen before or during the production process, which indicates that the surface is seriously polluted. Rust must be removed before the equipment is put into use, and the thoroughly cleaned surface shall be inspected by iron test and/or water test.

Rough grinding and machining

Grinding and machining will cause rough surfaces, grooves, overlaps, and burrs. Each kind of 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 surfaces can become the birthplace of corrosion and deposition products. Before re-welding, cleaning up weld defects or removing excess weld reinforcement height cannot be used for grinding. In the latter case, fine abrasives should be used for grinding.

Welding arc marks

When the welder strikes an arc on the metal surface, it will cause rough surface defects. The protective film is damaged, leaving a potential source of corrosion. The welder should start the arc on the welded bead or on the side of the weld joint. Then melt the pilot arc trace 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) has no spatter. However, when using GMAW (gas shielded metal arc welding) and FCAW (arc welding with flux core) two welding processes, if the welding parameters are used improperly, it will cause a lot of spatter. When this happens, the parameters must be adjusted. If you want to solve the problem of welding spatter, spray anti-splash agent should be applied to each side of the joint before welding, which can eliminate the adhesion of spatter. After welding, the splash-proof agent and various spatters can be easily cleaned away without damaging the surface or causing slight damage.

Flux

The welding process using flux includes 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.

Welding defects

Welding defects such as undercut, incomplete penetration, dense pores and cracks not only reduce the firmness of the joint, but also become a source of crevice corrosion. To improve this result, when cleaning operations, they also entrain solid particles. 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 local corrosion. Since these substances can act as a barrier, they will affect the chemical and electrochemical cleaning effect, so they must be thoroughly cleaned up. ASTM A380 has a simple water break (WATERBREAK) test to detect organic pollutants. During the test, water was poured from the top of the vertical surface. During the downward flow, the water would separate around the organic matter. Flux and acid chemical cleaning agent can remove oil and grease.

Residual adhesive

When the tape and protective paper are removed, a part of the adhesive will always remain on the stainless steel surface. If the adhesive is not hard, it can be removed with an organic solvent. However, when exposed to light and/or air, the adhesive hardens and forms a source of crevice corrosion. Then it needs to be cleaned mechanically with fine abrasives.

Paint, chalk and marker pen print

The effects of these pollutants are similar to those of oil and grease. It is recommended to use a clean brush and clean water or alkaline detergent for washing, or use high-pressure water or steam to rinse. The stainless steel whose structure is mainly ferrite in use. The chromium content is between 11% and 30%, and it has a body-centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This kind 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 resistance. , Water vapor, water and oxidizing acid corroded parts. This type of steel has disadvantages such as poor plasticity, significantly reduced plasticity and corrosion resistance after welding, 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, which makes this type of steel widely used.