Anodized steel is steel with a protective coating to prevent corrosion. Anodizing steel involves coating it with other types of anodized metal surface layers based on zinc oxides, aluminum, or other barrier compounds. An effective method is to react it with potassium or sodium hydroxide to create a layer of magnetite or dichroic magnetite. Anodizing steel with aluminum is not viable due to galvanic corrosion. Stainless steel and aluminum can be anodized together, but trace amounts of chloride can cause defects.
Anodized steel is steel that has been given a protective coating to strengthen it and retard the effects of corrosion. While anodized steel may look like many other anodized metals, especially aluminum, it’s not an actual anodizing process that creates the steel coating. This is because anodizing involves oxidizing the surface layer of the metal itself, which is often done with aluminum to produce a protective surface layer of aluminum oxide. When steel is oxidized, however, the result is a coating of ferric oxide, Fe2O3, more commonly known as rust, which offers little or no protection to the underlying metal and can, in fact, increase the likelihood of the underlying metal . corrode. The method used to create anodized steel products, therefore, involves coating the metal with other types of anodized metal surface layers based on zinc oxides, aluminum or other barrier compounds.
A particularly effective method of creating anodized steel is to react it with potassium hydroxide, KOH, or sodium hydroxide, NaOH. The use of these chemicals causes a layer of magnetite, Fe3O4 or dichroic magnetite to grow on the surface which offers protection to the underlying steel. While magnetite itself is blue-black in color, dichroic magnetite has an optical effect where a rainbow of colors reflect off the surface depending on the position from which it is viewed. Often, anodized steel cookware will exhibit this rainbow effect or other anodized products that have some aesthetic value. Although magnetite is chemically closely related to ordinary rust, which is sometimes composed of the compounds lepidocrocite, γFeOOH, or goethite, αFeOOH, it has much tougher and more protective qualities than rust.
Another method used to create anodized steel is to coat it with zinc or aluminum oxides. Various types of acids are used in an electrolytic bath to create oxides of the coating metals, from chromic acid to sulfuric acid and boric-sulfuric acid. The steel part serves as the negative anode part of the electrical circuit in the electrolyte and the donor metal such as zinc or aluminum forms the positive cathode. When current passes through the solution, this, together with the acid base, acts to remove the metal ions from the cathode and deposit them on the anode.
One of the problems with creating anodized steel is that it’s a noble metal that gets bonded in the process to metals like aluminum that aren’t noble. Because these metals have different corrosion potentials, it is common for the base metal to develop a galvanic layer between the two when bonded together. Galvanic corrosion rate is based on the overall surface area where the two metals meet and how passive or active their corrosion rates are relative to each other.
The only commercially viable process, therefore, in which anodized steel is produced by coating it with another elemental metal is that of stainless steel and aluminum. This is because ordinary steel suffers from a galvanic corrosion effect that occurs with aluminum when anodizing is attempted, and this prevents the metals from forming a strong bond. Galvanic corrosion is even more of a problem with bonding aluminum with metals such as copper, bronze and brass, so these metals are typically not anodized together. Another issue that can inhibit the anodizing process, even if stainless steel is paired with aluminum, is if trace amounts of chloride contaminate the process. Such contamination will also cause severe galvanic defects and make the anodized coating unreliable.
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