Rust Corrosion

How to Protect Against Corrosion

Posted on by chemical dynamics

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In 2013, the direct cost of corrosion was 3.1% of the 15.1 trillion in U.S. GDP, which in June 2013 is estimated to equal $500.7 billion. Corrosion is a an electrochemical process where the metal is oxidized by virtue of interaction with its environment, which results in the metal returning to its most stable oxidative state. This article will discuss those factors that influence corrosion, especially in regard to the use of coatings designed to protect the metal to which they’re applied. Accordingly, consideration of the fundamental factors that influence corrosion processes as it relates to the use of organic coatings will be considered herein.

Metals desire to be in their most thermodynamically stable state, which, in simplified terms, is the naturally occurring state of matter in its lowest energy state. Metals ordinarily exist naturally as oxides (e.g. iron oxide, aluminum oxide, zinc oxide etc.), because oxides represent their lowest energy state. Oxidation occurs at the anode (positive electrode) and reduction occurs at the cathode (negative electrode). Corrosion is normally accelerated by the presence of water, oxygen, and salts (particularly of strong acids).

Figure I – Corrosion of Common Metals
Figure I – Corrosion of Common Metals

Figure I lists a series of metals and their ability to resist corrosion. The most common metals used in industry include steel (cold rolled and hot rolled steel), aluminum, galvanized steel (hot dip and electrogalvanized steel) as well as galvalume. The latter two metal substrates utilize either a zinc layer or an aluminum/zinc layer respectively on the surface of the steel to enhance corrosion resistance.

Even though aluminum and zinc are less noble than steel, when not coated with an organic coating, they provide longer-term improved corrosion resistance than steel. When steel rusts, the corrosion product (ferric oxide) is loosely attached to the surface, whereas in the case of aluminum or a zinc/aluminum alloy, the corrosion products form a more tightly knit adherent layer to the metal surface that decreases the subsequent rate of corrosion (Table III).

Table III – Corrosion Loss of Uncoated Metals in microns/year in Various Environments. Exterior Durability of Organic Coatings, Eric V. Schmid, FMJ International, 1988
Table III – Corrosion Loss of Uncoated Metals in microns/year in Various Environments. Exterior Durability of Organic Coatings, Eric V. Schmid, FMJ International, 1988

 

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Roof

Identifying Roof Failure Points

Posted on by chemical dynamics

Chemical Dynamics has several decades of experience in the determination of the causes of coating failures and the determination of probable cause of exterior coating failures. Such roof failures may include corrosion, paint peeling, color change, severe chalking, cracking and spotting of coatings and damage due to scratches.  Normally coated metal roofs will demonstrate accelerated failure rates versus building sidewalls, as exposure to the sun, wind and precipitation is more severe.

Failures may be caused by one or more of the following issues: poor cleaning and/or poor metal pretreatment, poor quality metal, paint degradation (pigment and/or resin) of either or both primer and topcoat, paint formula defect, inadequate cure of coating, painting defects, paint contamination, damage and scratches during handling and building erection, and lastly aggressive climate conditions (chemical attack or environmental issues such as acid rain or a corrosive environment).

We have provided services including failure mode analysis, building site reports, test reports, analysis, expert witness reports, recommended failure remediation, and testimony on behalf of building owners as well as for companies that supply building materials.