The Main Problem With Stainless Steel

Stainless steel corrosion
Metal surfaces exposed to corrosive environments are subject to electrochemical or chemical reactions. The parts with weak corrosion resistance in the passivation film on the surface of stainless steel form pitting reaction due to self-excitation reaction, forming small pores, coupled with chloride ions, forming a strong corrosive solution and accelerating the speed of corrosion reaction. There is also intergranular corrosion cracking inside the stainless steel, all of which have a destructive effect on the passivation film on the surface of the stainless steel. Therefore, the stainless steel surface must be cleaned and maintained regularly to maintain its gorgeous surface and prolong its service life. When cleaning the surface of stainless steel, it is necessary to pay attention to the phenomenon of surface scratching, avoid using bleaching ingredients and abrasive washing liquid, steel wool balls, grinding tools, etc., in order to remove the washing liquid, rinse the surface with clean water after washing.

Stainless steel is a stable chromium-rich oxide film (protective film) that forms on its surface and is extremely thin and strong. Prevent oxygen atoms from continuing to infiltrate and continue to oxidize, and obtain anti-rust ability. If for some reason, the film is constantly damaged, and the oxygen atoms in the air or liquid will continue to precipitate out, forming loose iron oxide, and the metal surface will be constantly corroded.
For cost considerations, many domestic manufacturers have reduced chromium and nickel and increased the content of manganese in stainless steel. Experts believe that stainless steel is stainless because of the presence of chromium and nickel, and reducing the content of these two components will reduce rust resistance.

Precautions in the process of stainless steel machining:
Processing area: The processing area of stainless steel parts should be relatively fixed. Protective measures should be taken on the platform in the processing area, such as laying rubber mats, etc. When processing stainless steel parts, the damage to the surface protective layer of stainless steel parts should be avoided.

Chloride ions are present in the environment of use
Chloride ions are widely present, such as table salt/sweat stains/sea water/sea breeze/soil, etc. In the presence of chloride ions, stainless steel corrodes very quickly, even more than ordinary mild steel. Therefore, there are requirements for the use environment of stainless steel, and it needs to be wiped frequently, dust removed, and kept clean and dry. (This would give him a "misuse" label.) United States example: an enterprise uses an oak container to contain a solution containing chloride ions, which has been used for nearly 100 years, and was planned to be replaced in the nineties of the last century, because the oak material is not modern enough, and the container leaks due to corrosion 16 days after replacing it with stainless steel.
It is not solution-treated
The alloying elements are not dissolved into the matrix, resulting in low alloy content and poor corrosion resistance in the matrix structure.

Innate intergranular corrosion
This titanium- and niobium-free material has a tendency to intergranular corrosion. The addition of titanium and niobium, combined with a stabilization treatment, can reduce intergranular corrosion. A kind of high-alloy steel that can resist corrosion in the air or chemical corrosive medium, stainless steel is a kind of steel that has an aesthetically pleasing surface and good corrosion resistance, does not have to go through surface treatment such as color plating, and gives full play to the inherent surface properties of stainless steel, and is used in many aspects of steel, commonly known as stainless steel. Typical properties include 13 chromium steel, 18-8 chromium-nickel steel and other high-alloy steels. From a metallographic point of view, because stainless steel contains chromium, a thin chromium film is formed on the surface, and this film isolates the oxygen intrusive from the steel to resist corrosion. To maintain the corrosion resistance inherent in stainless steel, steel must contain more than 12% chromium. For applications where welding is required, the lower carbon content minimizes the precipitation of carbides in the heat-affected zone close to the weld, which can lead to intergranular corrosion of stainless steel in certain environments.

Dust
Production is often carried out in dusty areas, and there is often a lot of dust in the air, which is constantly falling on the surface of the equipment. They can be removed with water or alkaline solutions. However, adhered dirt and dirt need to be cleaned by high-pressure water or steam.

Floating iron powder or embedded iron
On any surface, free iron rusts and causes corrosion of stainless steel. Therefore, it must be cleared. Floating powder can generally be removed along with the dust. Some are very adhesive and must be treated with embedded iron. In addition to dust, surface iron can be used in a variety of sources, including cleaning with a regular carbon steel wire brush and shot peening with sand, glass beads or other abrasives previously used on carbon steel, low alloy steel or iron castings, or regrinding of non-stainless steel products mentioned above near stainless steel components and equipment. If stainless steel is not protected during unloading or lifting, the iron on the wire rope, spreader and work surface can easily become embedded or defile the surface. The presence of free iron can be prevented and detected by ordering requirements and post-fabrication inspections, and ASTM Standard A380 [3] specifies a rust test method for inspecting stainless steel surfaces for iron or steel particles. This test should be used when it is required that there 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 noted in Standard A380 [3], this test method is not recommended on the process surfaces of the equipment, i.e. the direct contact surfaces used in the production of human consumer products, if the rust test solution cannot be completely removed. The simple test method is to expose to water for 12~24 hours to check whether there are rust spots. This test is less sensitive and time-consuming. These are assay tests, not clean-up methods. If iron is found, it must be cleaned up using the chemical and electrochemical methods described below.

Scratches
In order to prevent the accumulation of process lubricants or products and/or dirt, scratches and other rough surfaces must be mechanically cleaned, usually with a special polishing machine for stainless steel. If the stainless steel is heated to a certain high temperature in the air during welding or regrinding, the chromium oxide hot temper color will appear on both sides of the weld, on the lower surface and at the bottom of the weld. The hot-tempered color is thinner than the oxidized protective film and is clearly visible. The color is determined by the thickness and can range from iridescent, blue, and purple to pale yellow and brown. Thicker oxides are generally black. It is caused by staying at high temperatures or at higher altitudes for long periods of time. When any of these oxide layers are present, the chromium content of the metal surface decreases, resulting in a decrease in corrosion resistance in these areas. In this case, it is not only necessary to eliminate the hot-tempered color and other oxide layers, but also to clean the chromium-poor metal layer underneath them.

Rust
Rust can sometimes be seen on stainless steel products or equipment before or during production, which indicates that the surface is heavily contaminated. Rust must be removed before the equipment is put into service, and thoroughly cleaned surfaces should be inspected by iron and/or water tests.