The inclusion of the element chromium contributes significantly to stainless steel’s corrosion resistance. Chromium, along with iron, nickel and small amounts of other metals present in the alloy, forms a tightly adhered complex oxide barrier on the metal surface that is known as the passive layer.
The passive layer is chemically resistant and forms in oxygenated circumstances. The metallurgical modification of this passive layer may provide significantly increased corrosion resistance. This is referred to as chemical passivation. To enhance the chromium-iron ratio, chemical passivation processes selectively eliminate iron atoms; however, certain common stainless steel techniques, such as welding, reverse the advantages of high chromium concentration.
Due to high temperatures, welding stainless steel may reduce its corrosion resistance due to phase changes. The two most detrimental phases of chrome carbides and the intermetallic phase sigma form at high temperatures. These changes occur between 8000 °F and 16000 °F.
They can also develop when the alloy is exposed to high temperatures outside of welding. Chrome carbides decrease the chrome in the base metal phase locally, while the sigma phase decreases the base metal’s local chromium and molybdenum content (found in 316 and stronger alloyed stainless steels).
The passive layer and the base metal underlying the passive layer become substantially more sensitive in these concentrated regions as a result.
Sensitization of stainless steel refers to the process of harmful carbide formation at elevated temperatures as when these carbides are present, the alloy becomes significantly more sensitive to corrosion attack in chemical conditions that would not be a concern for non-sensitized material.
Well Passivated Surface (Sketch). Image Credit: Astro Pak Corporation
Effect of Sensitization
Sensitization can be controlled through two methods. Using a solution annealing heat treatment, any carbides and sigma phases precipitated during welding can be dissolved back into the base metal phase. For 316 stainless steel, the typical solution annealing temperature is about 2000 ⁰F.
Effect of Sensitization. Image Credit: Astro Pak Corporation
Another way to prevent sensitized stainless steel is alloy selection. The stainless steels of the L family — 304 L and 316 L — have lowered carbon content compared to the original alloy with an identical numerical designation. Carbon reduction limits the driving power for sensitizing carbide phases to develop, meaning that more time at extreme temperatures is required than in a standard welding process.
Figure 1. Time-temperature-sensitization curves for type 304 stainless steel in a mixture of CuSO4 and H2SO4 containing free copper. Curves show the times required for carbide precipitation in steels with various carbon contents. Carbides precipitate in the areas to the right of the various carbon content curves. Image Credit: Astro Pak Corporation
Astro Pak has years of experience restoring stainless steel components subjected to severe metallurgical transformations. The team of experts at Astro Pak has developed electropolishing, proprietary pickling, and passivation processes that help restore sensitized welds or other near-surface imperfections that decrease corrosion resistance.
When choosing a corrosion-resistant alloy, higher surface chromium content is essential; however, it is vital to consider the processing steps to ensure that chromium can offer corrosion resistance.
Detrimental phases bind the chromium in the base metal, making local areas of the metal vulnerable to attack in an environment that should not affect a properly treated alloy. Welding can cause these harmful phases to form. Yet, there are several ways to prevent their formation or restore the alloy to an optimal useable condition after its formation.
This information has been sourced, reviewed, and adapted from materials provided by Astro Pak Corporation.
For more information on this source, please visit Astro Pak Corporation.