How to Reduce Corrosion of Metals

 

THE CORROSION BATTLE

 

The battle of corrosion is a constant fight because of the curse of all corrosion engineers – thermodynamics! The Second Law of Thermodynamics states, in simplified terms, that the naturally occurring state of metals is its lowest energy state, similar to humans on weekends. Metals ordinarily exist naturally as oxides (e.g. iron oxide, aluminum oxide, etc.) because oxides represent their lowest energy state. However, metal oxides are mined from the ground, and they are subjected to various unnatural acts – such as refining, casting, rolling, and forming into a variety of different shapes. These metals are now in an “activated” state, and do not want to stay there. They want to revert back to their naturally occurring state – oxides, or rust in the case of steel.

 

 

Metal failure due to corrosion is a ubiquitous and on-going problem for the general public globally.  It causes hundreds of millions of dollars of damage annually and compromises the safety, environmental, and appearance characteristics of the affected metal. When that occurs, I am often contacted by attorneys representing clients whose metal has failed in service to act as expert witness and determine why the metal has failed.

 

WAYS TO MITIGATE CORROSION OF METALS

 

There are several ways to minimize the corrosion of metals.  The most obvious, though not usually possible or practical, is to eliminate the causes of corrosion such as exposure of the metal to corrosive environments such as harmful chemicals, pollution, moisture, oxygen, high heat, etc.  If eliminating the source of corrosion is not possible, as in most instances, there are several practical ways to reduce the corrosion of metals.  These include: 

1. Protect the metal from the corrosive environment through the use of metal and/or paint coatings – this is the most common approach.
2. Avoid having dissimilar metals in contact with each other.  Dissimilar metals which are electrically coupled by simple contact drive galvanic corrosion.  Galvanic corrosion causes corrosion of the less corrosion-resistant metal to be increased, and corrosion of the more corrosion-resistant metal to be decreased, compared with the behavior of those metals when they are not in contact.  A common example of galvanic corrosion is when an inexperienced plumber uses copper water piping with a steel valve.  Corrosion of the steel valve is accelerated, while corrosion of copper piping is reduced, compared the corrosion of the copper and steel if they were exposed to the same environment but not connected to each other.
3. Minimize defects on the metal surface.  Defects on the metal surface such as grinding or polishing marks, mill roll marks, nonmetallic inclusions, oxides, grain boundaries, nicks, and scratches are high energy sites which can drive corrosion reactions.  Remember that metals like low energy sites, not high energy sites.   
4. Minimize residual salts and chemical impurities on the metal surface.  Metal processing or the cleaning and pretreatment steps prior to applying a coating can leave undesirable contaminants on the surface.  These impurities contribute to corrosion.
5. Consider corrosion when designing metal parts or structures; for example, by avoiding crevices, particularly those less than 0.004 inch.  Small crevices are prone to retention of moisture and other corrodents which accelerate corrosion.

 

ABOUT THE AUTHOR

Robert A. Iezzi, Ph.D., is founder of RAI Technical Solutions®, Inc. (www.rai-technical-solutions.com), a technology company that provides expert witness and consulting services on corrosion, metal coatings, paint coatings, surface preparation/pretreatments for metals and plastics prior to painting, and plastics.

 

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