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F-Files: Mechanisms of Failure
The case of…bubbles, and pinholes, and blisters, oh my!

From JPCL, September 2013

More items for Quality Control

by James D. Machen, PCS, KTA-Tator, Inc.
Richard Burgess, KTA-Tator, Inc., Series Editor

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Tagged categories: Coating failure; Failure analysis; Moisture-cured polyurethane; Polyurethane; Power Plants

Comment from Harry Peters, (11/8/2013, 11:24 AM)

Emissions from coal-fired power generation plants are prevalently sulfur oxides which convert to sulfates when in contact with atmospheric moisture and not sulfides, since the sulfur containing coal is being burned (oxidized) to generate the power. Mild sulfuric acid "acid rain" deposits on steel, inducing the potential sulfate induced corrosion. The MCU Zn primer did exactly what it was supposed to as a sacrificial coating to protect the steel. It appears there was too much sulfate laden on the steel as the Zn reacted with the sulfate and subsequently reacted with atmospheric oxygen to form the white ZnO powder indicated, leaving the sulfate still present to corrode the steel. Given the environment, testing for nonvisible sulfates and removal during surface preparation would have prevented the coating failure.

Comment from William Feliciano, (11/8/2013, 3:18 PM)

I’m not certain that the sulfates are the only problem here. The MCU’s are notorious for outgassing, pinholing and bubbling, and i’ve seen many an MCU film wind up looking like swiss cheese once cured. Porosity of a topcoat in this situation can allow water and oxygen access to the primer, which can cause the Zn to "kick over" into ZnO powder. Though the Zn may be sacrificing itself as expected, it is doing it too soon and too quickly. This results in premature aging and/or failure of the system. Though MCU's are a good tool in the toolbox, they must be applied by competent contractors familiar with the technology. Specification, ambient conditions and recoat times must be respected just like with any other coating.

Comment from Car F., (11/25/2013, 12:36 PM)

Fascinating analysis and diagnosis,...thank you!!, I've learnt a lot this morning.

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