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Metal Corrosion - How Do You Choose The Right Preventive Coating?
By William Gunnar
We define metal corrosion as chemical (or electrochemical) reactions occurring between the metal and the environment, which causes a negative impact on the metal's surface properties.
So, what's the secret to making corrosion resistant coatings work? Easy, really. Once you understand the types of metal corrosion and their contributing properties.
Fretting-- visible through pits, grooves, or debris relating to oxidation, occurring at the asperities or "peaks" of contact surfaces. An example is vibration, which is the result of load in the presence of repeated relative motion.
Galvanic-- seen by pits, holes, or oxidation products, is the result of localized corrosion between two dissimilar metals. Here, one metal corrodes faster than it would alone, while the second metal corrodes slower than it would alone. Often, this type of corrosion is confused with coupled metals exhibiting increased corrosion based on differences in their electromotive force potentials.
Uniform-- manifested by uniform, measurable material loss over a larger surface area, which continues until failure.
Pitting-- is a localized form of corrosion. Often difficult to detect, this type is considered more dangerous to design against. Pitting corrosion is initiated by local chemical or mechanical damage of protective oxide films or non-metallic inclusions, theoretically forming a "battery" between an abnormal anode or cathode site.
Other forms of it such as Intergranular, Exfoliation, Stress Corrosion, and Hydrogen Embrittlement, can also result in localized cracking, severe loss in ductility, and increased residual tensile stresses. Failure, here, is typically catastrophic.
Now that you recognize the corrosion mechanisms, what are the best protective coatings?
Look at the environment. Consider temperature, chemical or atmospheric condition, time of exposure. Is the contact intermittent or continuous? Are the corrosives in liquid or vapor form?
You see, it's not just about finding a non-reactive coating. It is about creating a barrier. And that means eliminating surface porosity. Typically, the thicker the coating, the better the permeation strength and better its performance. This will be true for any coating choice. So, how do you choose the right corrosion preventive coating? Easy. Once you understand your actual mechanism for metal corrosion.
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Metal Corrosion
William Gunnar, a degreed scientist, researcher, engineer, and friend, has helped thousands of 'best of class' product designers and equipment manufacturers -- for nearly 20 years now -- surface engineer with coatings for success.
His publication, Industrial Coatings World, brings wisdom, honesty, leading edge information, and no nonsense advice to thousands weekly. Put together by some of the world's leading experts in coatings and manufacturing, his FREE newsletters are truly must-reads for those who want to be 'in-the-know'.
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