Causes of surface corrosion of stainless steel pipes
Stainless steel pipe can also be called acid-proof stainless steel pipe, is mainly composed of two pieces of stainless steel pipe and acid steel, stainless steel pipe can be either atmospheric corrosion resistance, resistance to chemical corrosion medium. It is commonly believed that when the chromium content in the pipe is greater than 12%, it is characterized by the stainless steel pipe. Stainless steel pipe according to the microstructure after heat treatment and can be divided into five kinds, namely ferritic stainless steel pipe, martensite stainless steel pipe, austenitic stainless steel pipe, austenitic, ferritic stainless steel pipe and precipitated carbide stainless steel pipe.
Stainless steel pipe products in the production process there will always be surface damage, defects, and some influence on the surface of the material, such as dust, iron powder, hot temper color and other oxidation layer, rusty spot, grinding burrs, arc welding spot, welding spatter, welding flux, welding defects, oil and grease, residual adhesive and paint, chalk and marker and printing, and so on. Most of it is due to ignoring these harmful effects or not doing well. But these factors can damage the protective coating, which is corroded when the protective membrane is damaged and thinned. Also, corrosion usually does not involve the entire surface of the stainless steel pipe, only around the defect or around it. This kind of local corrosion is usually a pitting or crevice corrosion, which can be corroded in depth and breadth without affecting most surfaces. Here are the causes of these problems.
The stainless steel pipes are often in dust in the production process, and the dust in these air is constantly falling on the surface of the stainless steel pipe. The dust can be washed away by water or alkaline solution. But if there is a adhesion dust, use a water cannon to clean it up.
An iron powder or embedded iron
On the surface of the stainless steel pipe, the free iron will rust and corrode the stainless steel pipe. So be sure to get rid of it. The powder is usually cleared with the dust. Some of the adhesion is stronger and must be treated with embedded iron. Iron dust in addition to iron, stainless steel pipe surface source of many, including using ordinary carbon steel wire brush to clean up and used in carbon steel, low alloy steel or cast iron pieces of the used sand, glass beads or other abrasive shot peening treatment, or in the vicinity of a stainless steel pipe parts and equipment of the previously mentioned the grinding stainless steel pipe products. Next material or hanging if not stainless steel pipe protection measures, in the process of wire rope, sling and iron on the surface of the table also easier to embed or defiled with stainless steel pipe surface.
In order to prevent the lubricant process or product or dirt storing up, be sure to scratches and other rough surface of stainless steel pipe to do mechanical cleaning, make it smooth recovery.
Heat reheat and other oxidation layers
In the process of welding or grinding, stainless steel pipe in the air will be heated to a certain degree of high temperature and then weld on both sides, and under the surface and bottom of the weld will appear chromium oxide thermal heat tint. The heat return is thinner than the oxide film and is visible. This color is determined by the thickness of the color, which will present a rainbow, blue, purple and light yellow and brown color. Thicker oxide is usually black.
This is because of prolonged periods of high temperatures. When any type of oxidation layer occurs, the chromium in the surface of the stainless steel pipe decreases, resulting in less corrosion resistance in the corresponding area. In this case, not only to remove the heat and other oxidation layers, but also to clean up the poor chrome-metal layer below them.
Sometimes the rust on the stainless steel pipe or equipment indicates that the surface of the pipe is seriously polluted. The equipment must be removed before it is put into use, and the cleaned surface should be tested by iron test and water test.
Rough grinding and machining
Grinding and machining can lead to rough surface of stainless steel pipe, which can be used as a groove, overlap and burr. And each type of defect can also lead to the metal surface damage to a certain depth, so that the damage of the surface of the stainless steel pipe can’t through pickling, electrolytic polishing and shot peening method. Rough surface of the stainless steel pipe will become the birthplace of erosion and deposition products, heavy cleaning before welding defects or remove excess weld, now can’t use coarse grinding grinding, applying fine abrasive grinding.
Weld arc specks
Welders can cause surface roughness when they draw arc on metal surface. The protective membrane is damaged, leaving a potential source of corrosion. The welder shall draw the arc of the weld that has been welded or the side of the weld joint. The arc trace is then fused into the weld.
Welds have a lot to do with welding technology. If the gas is shielded from tungsten electrode welding or inert gas protection tungsten electrode welding is not splattered. However, if the welding parameters are not used properly, a large amount of splash will be formed if the welding parameters are not used when the welding process is shielded by the gas shielded metal arc welding and arc welding of the solder core. You must adjust the parameters at this point. If you want to solve the problem of welding spatter, apply the spray to each side of the joint before welding, which will eliminate the adhesion of the splash. After welding, it is easy to clean up the spray and all kinds of splashes without damaging the surface of the stainless steel pipe or just minor damage.
Use flux for welding process with manual welding, flux core arc welding and submerged arc welding, the welding process will leave fine flux grains in the surface of stainless steel pipe, the general cleaning method is not clear. These particles will be the source of corrosion for cracks, so be sure to remove the residual flux by mechanical cleaning.
Welding defects such as biting, unsoldered, denser pores and cracks not only reduce the fastness of the joints, but also become the source of corrosion. When you do this, you will also have solid particles. A good way to do this is by resoldering or resoldering.
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)