Corrosion in Stainless Steels
Stainless steels are a grade of steel alloy that contains a minimum of 12% chromium. The chromium readily forms a chromium oxide film on the steel surface that is sufficient to resist corrosion under a range of conditions. Stainless steels are not stain- or corrosion-proof, but rather are better suited to withstand staining and corrosion than are steels that do not contain any of the alloying elements that enhance corrosion resistance.
Even the highest quality stainless steel alloys may corrode under certain conditions:
- Pitting corrosion occurs when steel surfaces are deprived of oxygen, or when dissolved salts (e.g., chlorine) provide ions that compete to bond with the chromium. The lack of oxygen and/or competing ions prevent the formation of the protective oxide that allows the steel to resist corrosion.
- Pitting corrosion (along with crevice corrosion, discussed later) is most common in aqueous, chloride- containing solutions. Acidic conditions and elevated temperatures create optimal conditions for these modes of corrosion in stainless steels for food processing knives.
- Pitting corrosion can occur when
liquids or solutions (e.g., water, organic acids, cleaning solutions) bead on the stainless steel surface. These liquids chemically remove the protective oxide from the steel surface, allowing ions in the solution to attack the steel and produce corrosion (i.e., rust).
Pitting corrosion like this is typically the result of organic acids or water-soluble sanitation products being left in contact with the knife surface for extended periods. The acidic solutions penetrate the protective oxide and attack the steel underneath.
- Galvanic corrosion occurs when a galvanic cell is created between two different steel grades in the presence of an electrolyte solution, leading to the preferential corrosion of one grade. This typically occurs at points of contact between the two different types of steel (e.g., knives stored on galvanized hooks or carbon steel racks). The corrosion occurs in the less resistant grade of stainless steel (i.e., the grade of stainless steel with the lower chromium content). The effect can be prevented by placing electrical insulation between the materials (e.g., usingrubber/plastic sleeves and washers). Removing the electrolyte with thorough and proper cleaning, rinsing and drying of the knives and equipment can also prevent this type of corrosion.
- Contact corrosion is similar to galvanic corrosion, occurring where small particles of foreign material adhere to the steel surface. These particles are often generated by knife grinding or re-sharpening nearby, and create a small galvanic cell on the steel service that produces localized corrosion.
- Crevice corrosion occurs when a corroding agent is able to penetrate a confined space, but is relatively unable to escape. The corroding agent remains stagnant, and contact with the steel leads to corrosion. This mode of corrosion often occurs under gaskets, bolts, hubs, and in sharp corners.
Galvanic corrosion where a knife made with one grade of stainless steel grade remain in extended contact with fasteners made from a different grade of stainless steel, all while in the presence of an electrolyte solution.
- Rinse blades thoroughly with plain water after they have been in contact with cleaning solutions, food products, and other types of steel.
- Dry blades as thoroughly as possible after use and cleaning, and do not allow them to remain in contact with liquids/solutions for extended periods of time.
- Reduce knife contact with hubs, bolts, spacers, etc. made with dissimilar steels. Use rubber or plastic sleeves, washers and spacers to reduce the potential for galvanic corrosion
- Store knives to prevent contact with other metals or alloys (e.g. do not hang knives on galvanized hooks/nails, do not store them directly on steel shelving/racks, or store them in areas where machining work is done). Knives should be stored in use in a clean, protected area, away from processing areas or metal fabrication operations when not in use.\
Steel Selection and Corrosion
Corrosion resistance and wear resistance are inversely proportional for stainless steels. Corrosion resistance improves with an increase in chromium content, while wear resistance is enhanced with higher levels of carbon. In stainless steels, the addition of one element (chromium vs. carbon) typically comes at the expense of the other. The 400-series stainless steels are best suited for cutlery applications because they can be heat treated to provide an optimal combination of wear resistance, edge retention and blade life than other grades of stainless steel.