1. Dust

The production is often carried out in the site with dust. 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 solution. However, dust with adhesion needs to be cleaned by high-pressure water or steam.

2. Floating iron powder or embedded iron

On any surface, free iron will rust and corrode stainless steel. Therefore, it must be cleared. 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 brush, shot peening with sand, glass beads or other abrasives previously used on ordinary carbon steel, low alloy steel or cast iron, or grinding the non stainless steel products mentioned above near stainless steel parts and equipment. If the stainless steel is not protected during blanking or lifting, the steel wire rope, spreader and iron on the worktable are easy to be embedded or stain the surface.

Ordering requirements and post fabrication inspection can prevent and find the existence of free iron. ASTM standard A380 specifies the rust test method for inspecting iron or steel particles on the surface of stainless steel. This test method should be used when iron is not required. If the results are satisfactory, the surface shall be washed with clean pure water or nitric acid until the dark blue disappears completely.

As pointed out in standard A380, if the rust test solution cannot be completely removed, this test method is not recommended on the process surface of the equipment, that is, the direct contact surface used to produce human consumer goods. A relatively simple test method is to expose to water for 12-24 hours and check for rust spots. This test has poor sensitivity and time-consuming. These are detection tests, not cleaning methods. If iron is found, it must be cleaned by 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 (e.g. dry blasting, glass beads for abrasives).

4. Heat recovery color and other oxide layers

If the stainless steel is heated to a certain high temperature in the air during welding or grinding, chromium oxide heat recovery color will appear on both sides of the weld, the lower surface and bottom of the weld. The heat recovery color is thinner than the oxidation protective film and is obviously visible. The color depends on the thickness and can be seen in rainbow, blue, purple to light 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 such oxide layer appears, the chromium content on the metal surface will be reduced, resulting in the reduction of corrosion resistance in these areas. In this case, not only the heat recovery color and other oxide layers shall be eliminated, but also the chromium poor metal layer below them shall be cleaned.

5. Rust spots

Rust on stainless steel products or equipment is sometimes seen before or during fabrication, which indicates that the surface has been seriously polluted. The 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 rough surface, leaving defects such as grooves, overlaps and burrs. Each defect may also damage the metal surface to a certain extent. So that the damaged metal surface cannot be cleaned by pickling, electropolishing or shot blasting (such as dry sand blasting, abrasive glass beads). The rough surface can become the source of corrosion and deposition products. The weld defects or excess weld reinforcement can not be ground by rough grinding before rewelding. In the latter case, it shall be ground with fine abrasive.