（Please click the link for video：https://www.youtube.com/watch?v=nANNkLPygyg&t=12s）
304 stainless steel itself is a very good corrosion-resistant metal, but why does it rust?
In fact, under certain conditions and environments, there will be "accidental corrosion". Our company's internal sales statistics show that under the condition of selling 8000 square meters of metal mesh per day. There will be less than 10 rust feedbacks a year, and most of them occur in coastal areas. Strangely, in the past two years, the cases of stainless-steel mesh rusting from customers are more frequent than ever.
Because our stainless-steel materials are purchased from the domestic regular large factory, Baosteel Group, and regularly sampled for inspection. We have conducted various tests including salt spray and weather resistance can ensure that its composition and corrosion resistance meet the requirements of national standards.
In addition, we also adheres to the principle of "high production standards and high-quality raw materials", so our products are frequently rusted, which is really unacceptable. In this case, we immediately sent technicians to the scene. However, when we saw the screen, our inner doubts increased rather than decreased, because it was totally different from the ordinary stainless-steel rust.
We found that the so-called rust spots are distributed in a straight line from the top to the bottom. Also some of them are concentrated in the middle and lower parts of the screen, or near the frame of the screen, and white crystals can be clearly seen at the edge of the screen as well.
First, we cut off the suspected rusty part and use tools to peel off the coating. After observation with a high-power electronic magnifying glass, we couldn’t find the traces of stainless-steel wire rust. After further negotiation with the customer, we decided to disassemble the whole screen for analysis. When we pried off the cover of the screen, we were surprised to find that a large number of white crystals were piled up under the cover. Then we continue to remove the mesh from the screen and find that bulges, or even damages were appeared at the position where the mesh are placed. And these white objects come out of the damaged area.
In order to determine what these white substances are, we sent the samples to the quality inspection center for professional engineers to test and analyze. After testing, we found that these white crystals are actually aluminum oxides. We also found that some of the screws used to fix the mesh and the corner pieces used to assemble the screen were oxidized and corroded to varying degrees.
Why does this happen? After consulting industry professionals, we communicated with experts in materials science and finally reached the following conclusions based on relevant literature. First of all, due to the tense environmental protection situation in the past two years, the processing flow of al-alloy profiles that originally needed pickling and chromizing has been simplified. Therefore, the requirements for coating on the anti-corrosion performance of profiles have become stricter.
In the atmosphere, there are many dust, soot, iron powder, sulfide and nitric acid compounds floating, affected by the moisture in the air and rain, they will become corrosive aqueous solutions. After invading the surface of aluminum plate, there will be white corrosion pitting and perforation corrosion pitting. When aluminum and its alloys are in contact with dissimilar metals for a long time, we can see some contact corrosion. For example, compared with copper and iron with high potential, aluminum with low potential and metal materials with high potential are placed in the electrolytic solution in the form of electrical combination, which will cause corrosion due to battery action.
In the door and window industry, many accessories are made of stainless steel, copper, galvanized and other materials with higher potential than aluminum, which will accelerate the corrosion of al-alloy profiles.
Based on the above analysis, we can conclude that the customer's reaction before is not the rust of the stainless-steel mesh. Its caused by the oxidation corrosion of al-alloy profiles, also the corrosion of some screws fixing the mesh is particularly serious, and the oxidation solution seeps out. Then it flows down the screen, so what we see is a straight line of breakpoint type. It may also occur that the profiles on both sides have also been corroded, and then the oxidation liquid flows to the middle of the mesh with the mesh surface. Finally leaving a rust spot-like traces on it.
Later, we also discussed with the technical directors of many door and window factories. At present, it is quite difficult to eliminate this problem. The best solution is to delay its occurrence. So what can we do? We made the following recommendations.
First, adopt al-alloy profiles with excellent surface treatment, which can reduce the corrosion of the screen.
Second, we recommended to adopt the structure of drain hole to keep the profile dry.
Because once the coating of the profile is damaged, the corrosive aqueous solution in the environment will accelerate the decomposition of the protective film of the profile. Tiny holes are formed on the surface of profiles to accelerate the dissolution of al-alloy. In addition, when these acidic corrosion resultants contact with hardware fittings, they will also accelerate the corrosion of hardware.
Third, we can adopt the anti-rust agent at the place where the profile contacts with the hardware to isolate air and water.