Rusty stainless steel11/13/2023 ![]() Electropolishing attacks peaks and rounds valleys on material surfaces and raises the proportion of chromium at the surface. This process removes heat tint and other contaminants, including more iron and nickel, leaving surfaces rich in chrome. Note that these pastes have acids that can be hazardous to the environment and the operator.Īnother method involves electrochemical cleaning and polishing. Methods include treating material with acid solutions, including pickling pastes and gels, which clean the metal surface of free-iron contaminants. However, because of safety, budgetary, and environmental constraints, not all are suitable for fabricators. Several cleaning methods exist to enable passivation. Eliminating free iron requires a chemical treatment with harsh acids. Grinding, although an effective method to remove heat tint, leaves traces of free iron, which cause pitting and corrosion. Sandblasting removes heat tint but embeds contaminants in stainless steel. Heat tint is a serious contaminant that must be removed from the surface, not only for aesthetic reasons but also to allow stainless steel to self-passivate.Ī technician electrochemically cleans a circumferential weld joint on a rolled section of stainless steel to ensure complete passivation occurs. This leaves the metal just below the surface with a lower chromium level, which can negatively affect corrosion resistance. As heat tint colors are formed on stainless steel, chromium is drawn from below the surface of the metal to form a chromium-rich oxide surface layer. Heat tint is a thickening of the naturally occurring oxide layer on the surface of stainless steel. Welding creates a heat-affected zone and heat tint in which the alloy structure of stainless steel is altered. The chromium oxide microcoating acts as a barrier between the iron-dense alloy and the ambient air. A clean surface allows the chromium to react with oxygen in ambient air to form an inert, or passive, layer of chromium oxide on the metal’s surface. ![]() To passivate, stainless steel surfaces must be free of contaminants like free iron. Yet if steel tools are used on such metals, trace amounts of free iron (ferric material) can be left on the surface, and the iron will corrode. Exposed surface atoms readily react with oxygen in ambient air to form a stable layer of passive metal oxide. Some metals such as gold and titanium are self-passivating. Added elements such as nickel and molybdenum can impart other useful properties like enhanced formability and increased corrosion resistance. Stainless steels also contain varying amounts of carbon, silicon, and manganese. Increasing the amount of chromium increases resistance to corrosion. But passivation occurs only if the proportion of chromium is high enough and oxygen is present. Stainless steel contains enough chromium to undergo passivation by forming an inert film of chromium oxide on its surface. This iron oxide film (rust) is “active” and accelerates corrosion by making it easier for more iron oxide to form. Unprotected carbon steel rusts when exposed to air and moisture. Stainless differs from carbon steel by the amount of chromium present. This is why passivation is the final step in stainless steel parts manufacturing. It’s not uncommon for the passive oxide layer of stainless steel to sustain damage through any number of mechanical, industrial, and environmental processes. In stainless steel it typically takes 24 to 48 hours to achieve a uniform and stable passive layer, but passivation can occur only in certain conditions. It begins immediately after surface contaminants are completely removed. Passivation is an essential process in the manufacture and quality assurance of varying grades of stainless steel. Chromium produces a thin layer of oxide on the surface of the steel-the “passive” layer-that prevents surface corrosion. Stainless steel is an alloy of iron with a minimum of 10.5 percent chromium. What makes stainless steel stainless? In a word, chrome. In chemistry and engineering, passivation refers to a material becoming passive, or less affected by the environment. Rust is bad for business, which is why passivation is essential. For customers, it can mean headaches and expensive delays. Yet it’s not unusual for fabricators to complain about the appearance of rust after commissioning or installing components. In the world of stainless steel fabrication, stainless steel should mean just that-stainless. An automated arm electrochemically cleans to ensure complete passivation occurs along a stainless steel weld joint.
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