Stainless steel corrosion

An electrochemical reaction or chemical reaction occurs on a metal surface exposed to a corrosive environment, and is uniformly corroded. The part of the stainless steel surface passivation film with weak corrosion resistance forms a pitting reaction due to the self-excited reaction, and generates small pores, and the chloride ions are close to form a strong corrosive solution to accelerate the corrosion reaction rate. There is also intergranular corrosion cracking inside the stainless steel, all of which destroys the passivation film on the stainless steel surface. Therefore, the stainless steel surface must be regularly cleaned and maintained to maintain its gorgeous surface and extend its service life. When cleaning the stainless steel surface, care must be taken not to cause surface scratching. Avoid using bleaching ingredients and abrasive washing liquid, steel balls, grinding tools, etc., in order to remove the washing liquid, rinse the surface with clean water after washing. 

Stainless steel is a very thin, strong and dense, stable chromium-rich oxide film (protective film) formed on its surface. Prevent oxygen atoms from continuing to infiltrate and continue to oxidize to obtain corrosion resistance. Once there is a reason for this, the film is continuously destroyed, and the oxygen atoms in the air or liquid are continuously separated out to form loose iron oxide, and the metal surface is continuously rusted.

Many domestic manufacturers have reduced chromium and nickel in stainless steel for cost reasons and increased manganese content. Experts believe that the reason why stainless steel can be stainless is because of the presence of chromium and nickel, reducing the content of these two components will reduce the rust resistance.

Chloride ion in the environment of use

Chloride ions are widely present, such as salt/sweat/seawater/sea breeze/soil. Stainless steel in the presence of chloride ions, corrosion is very fast, even more than ordinary low carbon steel. Therefore, there is a requirement for the use environment of stainless steel, and it is necessary to wipe frequently to remove dust and keep it clean and dry. (This will give him a "use is not appropriate.") There is an example in the United States: a company uses a oak container to hold a solution containing chloride ions, which has been used for nearly a hundred years and is scheduled to be replaced in the 1990s. Because the oak material is not modern enough, the container leaks due to corrosion after 16 days of replacement with stainless steel.

Natural intergranular corrosion

Such titanium and niobium-free materials have a tendency to intergranular corrosion. The addition of titanium and niobium, together with a stable treatment, can reduce intergranular corrosion. A high-alloy steel that resists corrosion in air or chemically corrosive media. Stainless steel has an aesthetically pleasing surface and good corrosion resistance. It does not require surface treatment such as plating, but also exhibits the inherent surface properties of stainless steel. One of the aspects of steel, commonly referred to as stainless steel. Representative properties include high-alloy steels such as 13 chrome steel and 18-8 chrome nickel steel. From the perspective of metallography, because stainless steel contains chromium and the surface forms a very thin chromium film, this film is isolated from the intrusion of oxygen in the steel to resist corrosion. In order to maintain the corrosion resistance inherent in stainless steel, the 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 near the weld, which may result in intergranular corrosion of the stainless steel in certain environments.

Floating iron powder or embedded iron

On any surface, free iron rusts and corrodes the stainless steel. Therefore, it must be cleared. Floating powder can generally be removed with dust. Some adhesion is very strong 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 brushes and shot blasting with sand, glass beads or other abrasives previously used on plain carbon steel, low alloy steel or cast iron parts, or The aforementioned non-stainless steel products are ground near stainless steel parts and equipment. Wire ropes, spreaders and iron on the work surface can easily embed or smear the surface if the stainless steel is not protected during the blanking or lifting process. Ordering requirements and post-production inspections prevent and detect the presence of free iron. ASTM Standard A380 specifies the rust test method for the inspection of iron or steel particles on stainless steel surfaces. This test should be used when there is absolutely no iron available. If the results are satisfactory, wash the surface with clean pure water or nitric acid until the dark blue color completely disappears. As indicated in Standard A380 ,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, ie the direct contact surface used to produce human consumables. A relatively simple test method is to expose the water for 12 to 24 hours to check for rust spots. This test is poorly sensitive and time consuming. These are test tests, not cleanup methods. If iron is found to exist, it must be cleaned by chemical and electrochemical methods described later.

Scratch

In order to prevent the accumulation of process lubricants or products and/or dirt, mechanical cleaning of scratches and other rough surfaces must be performed by special stainless steel polishing machines. If the stainless steel is heated to a certain high temperature in the air during welding or grinding, the chrome oxide thermal tempering color will appear on both sides of the weld, the lower surface of the weld and the bottom. The thermal tempering color is thinner than the oxidized protective film and is clearly visible. The color depends on the thickness and can be seen in iridescence, blue, purple to pale yellow and brown. Thicker oxides are generally black. It is due to staying at a high temperature or for a long time at a higher degree. When any such oxide layer is present, the chromium content of the metal surface is reduced, resulting in reduced corrosion resistance in these areas. In this case, not only the thermal tempering color and other oxide layers are eliminated, but also the chromium-depleted metal layer underneath them should be cleaned.

Rough grinding and machining

Both grinding and machining can result in rough surfaces with defects such as grooves, overlaps and burrs. Each defect may also damage the metal surface to such a depth that the damaged metal surface cannot be cleaned by pickling, electropolishing or shot blasting (such as dry blasting, abrasive glass beads). Rough surfaces can be the source of corrosion and deposition products. Cleaning weld defects before re-welding or removing excess welds can not be ground with coarse grinding. In the latter case, it should be ground with a fine abrasive.

Welding splash

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 soldering) does not splash. However, when using two welding processes of GMAW (gas shielded metal arc welding) and FCAW (arc welding with flux core), if the welding parameters are used improperly, a large amount of splash will be caused. When this happens, the parameters must be adjusted. If you want to solve the problem of welding spatter, apply anti-splash agent on each side of the joint before welding, which can eliminate the adhesion of spatter. This anti-splash agent and various spatters can be easily removed after soldering without damaging the surface or causing minor damage.

The welding process using flux is manual welding, flux core arc welding and submerged arc welding. These welding processes leave fine flux particles on the surface, and ordinary cleaning methods cannot remove them. This particle will be a source of corrosion for crevice corrosion and must be removed by mechanical cleaning.

Welding defect

Welding defects such as undercuts, incomplete penetration, dense pores and cracks not only reduce the firmness of the joint, but also become a source of corrosion for crevice corrosion. These results are improved when they are cleaned and they entrain solid particles. These defects can be repaired by re-welding or re-welding after dressing.

Oil and grease

Organic substances such as oil, grease and even fingerprints can become sources of corrosion for localized corrosion. Since these substances act as barriers, they affect the chemical and electrochemical cleaning effects and must be thoroughly cleaned up. ASTM A380 has a simple WATERBREAK test to detect organic contaminants. During the test, water is poured from the top of the vertical surface, and the water separates along the periphery of the organic matter during the downward flow. Flux and/or acid chemical cleaner removes traces and grease.

Residual binder

When the tape and the protective paper are torn off, a part of the adhesive remains on the stainless steel surface. If the adhesive is not yet hard, it can be removed with an organic flux. However, when exposed to light or air, the binder becomes hard and forms a source of corrosion for crevice corrosion. Mechanical cleaning is then required with fine abrasives.

Paint pen

The effects of these contaminants are similar to those of oils and greases. It is recommended to wash with a clean brush and clean water or alkaline cleaner, or with high pressure water or steam. Stainless steel mainly composed of ferrite in use. The chromium content is between 11% and 30%, and has a body-centered cubic crystal structure. These steels generally do not contain nickel, and sometimes contain a small amount of elements such as Mo, Ti, Nb, etc. These steels are characterized by large thermal conductivity, small expansion coefficient, good oxidation resistance and excellent resistance to stress corrosion. Parts that are corroded by water vapor, water, and oxidizing acids. Such steels have the disadvantages of poor plasticity, post-weld plasticity and corrosion resistance, which limit their application. The application of refining technology (AOD or VOD) can greatly reduce the gap elements such as carbon and nitrogen, thus making this type of steel widely used.