Corrosion Fundamentals

• Corrosion Fundamentals
• Why Metals Corrode
• Electrochemistry
• The Nature of Matter
• Electrochemical Cells
• Forms of Corrosion
• Uniform Corrosion
• Galvanic Corrosion
• Concentration Cell Corrosion
• Pitting Corrosion
• Crevice Corrosion
• Filiform Corrosion
• Intergranular Corrosion
• Stress Corrosion Cracking
• Corrosion Fatigue
• Fretting Corrosion
• Erosion Corrosion
• Dealloying
• Hydrogen Damage
• Corrosion in Concrete
• Microbial Corrosion
• Corrosion Control
• Design
• Materials Selection
• Protective Coatings
• Inhibitors and Other Means of Environmental Alteration
• Corrosion Allowances
• Cathodic Protection
• Sources of Additional Information

This type of corrosion occurs under painted or plated surfaces when moisture permeates the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. Their use should be avoided unless absence of an adverse effect has been proven by field experience. Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality.

(Courtesy of www.cp.umist.ac.uk)

Filiform corrosion normally starts at small, sometimes microscopic, defects in the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. Their use should be avoided unless absence of an adverse effect has been proven by field experience. Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality.

The picture on the left shows filiform corrosion causing bleed-through on a welded tank. The picture on the right shows "worm-like" filiform corrosion tunnels forming under a coating at the Atmospheric Test Site.

Filiform corrosion is minimized by careful surface preparation prior to coating, by the use of coatings that are resistant to this form of corrosion (see above), and by careful inspection of coatings to insure that holidays, or holes, in the coating are minimized.