Flash oxidation, or rust, is often a problem either prior to or just after heat treatment. But what is rust, really? We attempt to answer that question here.
Rust is a form of corrosion. By definition, corrosion is the chemical or electrochemical reaction between a material – usually a metal – and its environment that produces a deterioration of the material and its properties. In general, corrosion becomes a problem when the appearance or functionality of a product is diminished.
The rusting of a metal is an electrochemical process involving an anode (a site where oxidation of a metal generates positively charged ions), a cathode (a site where reduction of oxygen and hydrogen in water consumes electrons), an electrolyte (a medium that connects the anode and cathode) and a conductor (a bridge that electrically connects the anode to the cathode; in other words, a mechanism by which the electrical circuit is complete).
Chemistry of Rust
Even small differences in electrical potential can cause the formation of reactive (anodic) sites on the surface of a part. Surface imperfections, grain boundaries, temperature gradients, stress and electrochemical polarization are typical trigger mechanisms. The formation of these sites causes electrons to leave the anode at the electrical contact with the cathode. The departing electrons turn the iron atoms into ferrous ions at the anode-electrolyte interface, resulting in disintegration of the anode (Equation 1).
(1) Fe → Fe++ + 2e-
These ferrous ions react with water to form ferrous hydroxide (Equation 2).
(2) Fe++ + 2H2O → Fe(OH)2 + 2H+
The ferrous hydroxide then reacts with oxygen to produce ferric oxide (the commonly recognized form of rust) and water (Equation 3).
(3) 4Fe(OH)2 + O2 → 2Fe2O3 + 4H2O
The combined anode equation is obtained by adding the above equations (Equation 4).
(4) 4Fe + 4H2O + O2 → 2Fe2O3 + 8 H+ + 8e-
The electrons then migrate from the anode toward the cathode, and they react with oxygen and water at the cathode-electrolyte interface, forming hydroxide ion (Equation 5).
(5) O2 + 2H2O + 4e- → 4OH-
The final result, combining Equations 4 and 5, is Equation 6.
(6) 4Fe + 2H2O + 3O2 → 2Fe2O3 + 8H2O
Note that in Equation 6 water is conserved and only oxygen and iron are consumed. Thus, once a corrosion cell has been formed, the formation of rust is maintained without the further addition of water (that is, only oxygen is required at this point). This is a point that is often confusing to customers and heat treaters alike.
For the heat treater, the presence of rust on the surface of a component part prior to heat treating can result in spotty hardness, improper case depth and/or incomplete transformation in the affected area. After the heat-treat operation is completed, rust in the form of flash oxidation is a cosmetic issue that raises concerns as to the ultimate reliability of the part.
- ASTM G15 (Standard Terminology Related to Corrosion and Corrosion Testing)
- Fuels and Lubricants Handbook: Technology, Properties, Performance and Testing,George E. Totten (Ed.), ASTM International 2003, Chapter 31: Corrosion by Maureen E. Hunter and Robert F. Baker, Corrosion
- Herring, Daniel H., Atmosphere Heat Treatment, Volume I, BNP Media, 2014
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