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Stainless steel is often chosen due to its excellent corrosion resistance, high strength, and/or attractive appearance. Moreover, unlike carbon steel, stainless steel has a passivation layer on its surface, which makes it less prone to oxidation or rusting. However, great care must be taken during initial production and subsequent welding processes to maintain the integrity of the passivation layer. Why choose expensive stainless steel? Stainless steel boasts excellent corrosion resistance, high-temperature oxidation resistance, and high strength, which is why it is widely used. A previous article discussed various types of stainless steel and outlined how, when using different welding techniques and assembling stainless steel parts, it is possible to avoid weakening the steel’s corrosion resistance, oxidation resistance, and mechanical properties, as well as how to prevent weld defects from occurring. The “rust-proof” property of stainless steel is due to the formation of a strong-adherent thin film of passivated nickel oxide and chromium oxide on its surface, which prevents further contamination of that surface. What is passivation? Passivation is a chemical treatment that uses a specific acid solution to remove free iron and other surface contaminants from the stainless steel surface, while promoting the formation of passive chromium oxide/nickel oxide layers to prevent further corrosion of the stainless steel. Passivation is a critical step in the production of stainless steel; it is necessary to carefully develop quality assurance plans and conduct appropriate inspections for various types of stainless steel. Passivation begins immediately after removing surface impurities (see Figure 1). It is a well-established fact that the passivated oxide layer of stainless steel can withstand various types of mechanical, industrial, and environmental damage. Therefore, passivation treatment is considered a key step in manufacturing stainless steel parts. Why does stainless steel also need passivation? Stainless steel differs from carbon steel due to its different chromium content. Unprotected carbon steel rusts when exposed to air and humid environments. This layer of iron oxide (rust) is \"active,\" and it continues to produce more iron oxide, thereby accelerating corrosion. Stainless steel contains sufficient chromium, which allows an inert chromium oxide film to form on its surface, resulting in passivation. However, passivation occurs only under conditions of sufficient chromium content and in the presence of oxygen. To achieve passivation, corrosive contaminants such as free iron on the stainless steel surface must be completely removed. In this way, chromium can react with the oxygen in the air to form an inert or passivated chromium oxide film on the surface of stainless steel. The resulting thin layer of chromium oxide acts as a barrier that completely separates the ferroalloy from the ambient air. Removing surface contaminants helps facilitate passivation. Welding creates heat-affected zones in metals and alloys, which can alter the (local) alloy composition of stainless steel. Temper coloring is the solidification phenomenon of the oxide layer on the surface of stainless steel. When a tempering color forms on stainless steel, chromium migrates out from beneath the metal surface, forming a chromium-rich oxide layer that reduces the chromium content in the alloy layer beneath the surface, thereby lowering its corrosion resistance. Temper color is a serious pollutant. To achieve an aesthetic appearance and to enable spontaneous passivation of stainless steel, it is necessary to remove the tempering color from the metal surface. Many cleaning methods contribute to the occurrence of passivation. However, due to safety, economic, and environmental constraints, not all methods are applicable. These methods include materials containing acidic solutions such as pickling pastes, which can be used to remove iron impurities from the metal surface. It should be noted that these pickling pastes contain acids, which may pose a risk to the atmosphere as well as to the people who work with them. Sandblasting can remove the tempering color, but it introduces impurities into the stainless steel ； Polishing is an effective method for removing the tempering color, but it leaves traces of free iron, which accelerates the occurrence of pitting and corrosion. Removing free iron requires chemical treatment with strong acids. Some weld cleaning and polishing methods use food-grade acids and electricity to eliminate the tempering color, thereby promoting the chemical passivation of stainless steel parts. ASTM International (American Society for Testing and Materials) has defined this. Standard methods for testing passivation effectiveness There are many methods for detecting passivation effectiveness, but it should be noted that not all testing methods are suitable for all types of stainless steel. ASTM International’s standards describe various testing methods; for example, ASTM-A380 outlines the best practices for cleaning, derusting, and passivating stainless steel parts, equipment, and systems. ASTM-A967: Describes the methods for detecting passivation, the acceptance criteria, and how to ensure the effectiveness of passivation. Water immersion test: This test involves immersing the passivated components in distilled water to detect impurities on the anode surface, including free iron. Salt spray test: This test uses a controllable corrosive environment to determine the corrosion resistance of stainless steel. During testing, the specimen is placed in a salt spray chamber filled with a 5% sodium chloride (NaCl) solution, and the chamber is heated to 95 degrees Fahrenheit. High humidity test: This test requires specialized testing equipment and a humidity chamber (to be placed in an environment with 97% (±3%) humidity and 100 (±5) degrees Fahrenheit temperature for at least 24 hours). The specimen must be immersed in acetone or methanol, and then dried in an inert atmosphere or a dehydrated container. Copper sulfate test: This test is used for specific steel grades such as austenitic, martensitic, ferritic, and precipitation-hardening steels with a chromium content of at least 16%. This test contains toxic substances, so it is not recommended for use in the food industry. Potassium ferrocyanide-nitric acid test: This test is recommended for detecting very small amounts of free iron on austenitic stainless steels (200 series and 300 series). The key point is that free iron on stainless steel acts as anodic contaminants; therefore, the corrosion of free iron on the surface of stainless steel occurs at a faster rate than that in unalloyed steel. Free iron corrosion can form a protective barrier on the surface of stainless steel, just as zinc corrosion after galvanizing forms a protective barrier on the surface of carbon steel. Another situation is when welding is carried out using flux-cored welding processes (such as submerged arc welding with flux-cored wires and gas shielded welding), rust can form on the surface of stainless steel. These fluxes are usually easy to remove or peel off on their own, giving the weld surface an appearance of cleanliness; however, a thin layer of iron-rich material always remains. If the surface of the welded metal is not polished or chemically treated after welding, this iron-rich layer is prone to rusting. The properties of this rust are very similar to those of the rust formed by the contact impurities discussed in detail above. Generally speaking, rust on the outer surface of stainless steel welds is not a major issue, but an unsightly appearance can also affect the overall look of the object. Therefore, stainless steel should be cleaned after welding, interactions with steel and iron should be avoided during manufacturing and assembly, and it should be confirmed that there are no iron impurities before it is put into use, so as to ensure that stainless steel items do not rust. If end-users want to ensure that the surfaces of the stainless steel products they purchase do not rust, it is necessary to be clear about the stainless steel cleaning methods mentioned above; in addition, inspectors must verify that welders have used the prescribed methods. Of course, end-users should also adopt appropriate management and storage methods to prevent stainless steel products from being contaminated due to their own actions.