Stainless steel’s corrosion resistance is derived primarily through the inclusion of the element chromium. Chromium, along with iron, nickel and small amounts of other metals present in alloy, form a tightly adhered complex oxide barrier on the metal surface called the passive layer. The passive layer forms in oxygenated environments and is quite resistant to chemical attack. Further metallurgical modification of this passive layer can provide even greater corrosion resistance. This process is called chemical passivation. Chemical passivation treatments selectively remove surface iron atoms to improve the ratio of chromium relative to iron. Some common processes to stainless steel, however, negate the benefit of high chromium content – one example is welding.
Welding stainless steel can reduce the corrosion resistance of the metal because of phase transformations occurring at elevated temperature. The two major detrimental phases that form at elevated temperature are chrome carbides and the intermetallic phase named sigma. These transformations occur roughly between 800⁰F and 1600⁰F. Further, they can occur anytime the alloy is subjected to this high of temperature, not just during a welding practice. Chrome carbides locally reduce the amount of chrome in the base metal phase while sigma phase reduces both the local chromium and molybdenum (present in 316 and stronger alloyed stainless steels) content of the base metal. As a result, both the passive layer and the base metal below the passive layer in these localized areas become much more susceptible to corrosion. This process of detrimental carbide formation at elevated temperature is called the sensitization of stainless steel because where these carbides exist, the alloy becomes much more sensitive to corrosion attack in chemical environments that would cause no concern to the non-sensitized material.
Effect of Sensitization
Sensitization is controlled in two ways. A solution annealing heat treatment can dissolve any carbides and sigma phases precipitated from welding back into the base metal phase. A typical solution annealing temperature for 316 stainless steel is around 2000⁰F. The other method in preventing sensitized stainless steel is through alloy selection. The L family of stainless steels, i.e. 304L and 316L have reduced carbon content than their original alloy of the same numerical designation. The carbon reduction significantly reduces the driving force for sensitizing carbides phases to form, and thus much more time at elevated temperature is required for formation than a typical welding operation would provide.
Astro Pak has several decades of experience in restoring stainless steel components subjected to the adverse metallurgical transformations. Our team of experts have created proprietary pickling, electropolishing and passivation processes that restore sensitized welds or other near-surface defects that reduce corrosion resistance. Reach out today to learn more about how Astro Pak can protect or restore your system through various chemical services.
Final Word
Higher surface chromium content is critical for selecting a corrosion resistant alloy, but consideration of the processing steps is required to ensure the chromium can provide its corrosion resistance. Detrimental phases tie up the chromium in the base metal, rendering local areas of the metal susceptible to attack in an environment that should be no issue to the properly processed alloy. Welding is one of the common processes for stainless steel that can form these detrimental phases, but options exist for preventing their occurrence or even restoring the alloy to an optimal serviceable condition after they have been formed.
