Cleaning of stainless steel surface damage and entrained foreign objects
1. Dust
The production is often carried out in dusty venues. There are often many specks of dust in the air, which are constantly falling on the surface of the equipment. They can be removed with water or alkaline solution. However, the adhesion of dirt requires high-pressure water or steam to clean up.
2. Floating iron powder or embedded iron
On any surface, free iron will rust and cause corrosion of stainless steel. Therefore, it must be cleared. The floating powder can generally be removed with dust. Some have strong adhesion and must be treated with 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 that have been used on plain 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 stainless steel, the steel wire rope, spreader and the iron on the work surface are easy to embed or contaminate the surface.
Order requirements and post-production inspection can prevent and discover the existence of free iron. ASTM Standard A380 [3] specifies the rust test method for inspecting the surface of stainless steel iron or steel particles. 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 stainless steel 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 simpler 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 with the chemical and electrochemical methods described later.
3. Scratches
In order to prevent the accumulation of process lubricants or products and/or dirt, 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 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 a 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.
5. Rust spots
Before or during the production process, we sometimes see rust on stainless steel products or equipment, which indicates that the surface is seriously contaminated. 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.
6. Rough grinding and machining
Grinding and machining will cause surface roughness, leaving grooves, overlapping and burrs, and other defects. 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. Rough surfaces can become the birthplace of corrosion and deposition products.
Before rewelding, 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.
7. Welding arc strike marks
When the welder strikes the arc on the metal surface, it will cause the surface roughness defect. 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 traces into the weld.
8. Welding spatter
Welding spatter has a lot to do with the welding process. For example, GTAM (Gas Shielded Tungsten Arc Welding) or TIG (Inert Gas Shielded 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, a large amount of spatter will be caused.
When this happens, the parameters must be adjusted. If you want to solve the problem of welding spatter, you should apply an anti-splash agent on each side of the joint before welding, which can eliminate the adhesion of spatter. After welding, this splash preventer and various spatters can be easily cleaned away without damaging the stainless steel surface or causing slight damage.
9. 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.
10. 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 corrosion for 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.
11. Oil and grease
Organic substances such as oil, grease, and even fingerprints can become a source of local corrosion. Because 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 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/or acid chemical cleaning agents can remove oil stains and grease.
12. 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 flux. 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.
13. Paint, chalk, and marking 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 cleaning agent for washing. You can also use high-pressure water or steam to rinse.
Stainless steel whose main structure is ferrite in the state of 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 stainless 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 stainless 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, so this type of steel is widely used.