KEY CONCEPTS
• Most metals are protected from corrosion through the formation of an oxide film upon exposure to air at room temperature.
• A new study now shows that metal oxide formation is a nonequilibrium process where a metal oxide solute dissolves within a different metal oxide solvent. 
• The solvent in a specific metal alloy is the lowest energy solid metal oxide. 

Minimizing corrosion continues to be a large challenge for the lubricant industry. A series of different types of inhibitors are used to minimize rusting on ferrous alloys and corrosion/staining of nonferrous alloys such as aluminum. 

One type of corrosion that is present when adjacent metal surfaces are in close proximity is crevice corrosion. A past TLT article describes how crevice corrosion was observed in real time using an instrument known as an electrochemical surface forces apparatus (1). At the onset of corrosion, the researchers reported that the metal surface became very bright when new pits suddenly formed.

While corrosion inhibitors can be effective, in most cases metal films are protected by oxide films formed upon exposure to air at room temperature. Laurence Marks, professor of materials science and engineering at Northwestern University in Evanston, Ill., says, “Almost every metal, with the exception of gold, produces a protective oxide film.”

Although corrosion has been studied for 100 years, Marks contends that researchers are still trying to gain a better understanding of this phenomenon. He says, “Corrosion is an older field that needs to be studied using modern techniques such as microscopic analysis and using theoretical modeling.” A better understanding of how oxide films are generated may lead to the development of stronger protective films leading to better metal resistance to corrosion. 

Marks contends that there are significant gaps in researcher’s understanding of the phenomenon of corrosion. He says, “The current model for oxide film formation states that the most stable metal oxide is formed initially. But most researchers have admitted that this model does not work. No explanation has yet been made about how complex oxides are formed with structures that cannot be predicted by the current model.”

Part of the problem is that the current view of metal oxide film formation is too simple. Marks says, “The assumption has been made that an oxide coating produced for an aluminum, nickel alloy consists of alumina and nickel oxide. But it is not that simple.”

Metal oxide film formation has been thought to occur under equilibrium conditions. Marks says, “In a classic chemical reaction, ‘A’ is converted to ‘B’ through a transition state where ‘A’, ‘B’ and the transition state are in equilibrium. The assumption is made that the process is reversible.”

But metal oxide film formation does not occur through this approach. A new theory has now been developed from experimental results and modeling that shows this phenomenon is best explained as a nonequilibrium process.

Metal solute
Marks and his colleagues determined through the use of experimentation and theoretical modeling that metal oxide film formation is a nonequilibrium process. He says, “Oxide formation occurs so fast on a time scale that atoms present in the oxide can move so that they do not have time to rearrange. The composition of the resulting film is controlled by the metal-oxide interface velocity.”

One way to look at metal oxide film formation is to view it as a metal oxide solute dissolving within a different metal oxide solvent. For a specific metal alloy, the solvent is the lowest energy solid metal oxide while the solute is the other oxide captured within it. 

Marks says, “An analogy to metal oxide film formation is to take sea water and freeze it very quickly. The salt present has no change to escape and must stick around in the ice. In contrast, if the sea water were allowed to freeze slowly, the salt would have the opportunity to escape.”

The researchers discovered this mechanism by using transmission electron microscopy and atom-probe tomography to evaluate the metal oxide films. A nickel, chromium, molybdenum alloy was studied. Marks says, “This alloy is used in a number of important applications such as jet engine turbine blades.”

The experimental analysis showed that the metal oxide film formed was not a combination of chromium oxide and nickel oxide but rather more complicated. Figure 2 illustrates how an oxide front (oxygen atoms are blue) moves at a specific velocity through the metal matrix consisting of nickel atoms in red, chromium atoms in green and molybdenum atoms in brown to produce the film. 


Figure 2. The formation of a metal oxide film moving at a velocity v on an alloy of nickel and chromium is shown. Oxygen atoms are in blue, nickel atoms are in red, chromium atoms are in green, and molybdenum atoms are in brown. (Figure courtesy of Northwestern University.)

Marks says, “Chromium oxide exhibits the lowest energy of any of the metal oxides and is the solvent in this case while nickel oxide is the solute. Molybdenum also is captured but is only present at small concentrations in the alloy.”

The researchers developed a model for predicting if this nonequilibrium solute capture effect can occur with specific metal alloys. Marks says, “We found that almost all elements in the first transition metal row of the periodic table can form oxide films in a nonequilibrium manner. The two criteria we used are the free energy change during oxidation of the metal surface must be negative and the metal-oxide interface velocity must be too fast for equilibrium to take place.”

Marks believes that this finding will enable researchers to design metal alloys that exhibit more robust oxide films and can better withstand corrosion. It also will help in designing alloys that can withstand both corrosion and sliding-tribocorrosion, another problem commonly found in the lubricant industry. Additional information can be found in a recent article (2) or by contacting Marks at l-marks@northwestern.edu. 

REFERENCES
1. Canter, N. (2017), “Observation of corrosion as it happens,” TLT, 73 (12), pp. 10-11.
2. Yu, X., Gulec, A., Sherman, Q., Cwalina, K., Scully, J., Perepezko, J., Voorhees, P. and Marks, L. (2018), “Nonequilibrium Solute Capture in Passivating Oxide Films," Physical Review Letters, 121 (14), 145701.