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

Dealloying is a rare form of corrosion found in copper alloys, gray cast iron, and some other alloys. Dealloying occurs when the alloy loses the active component of the metal and retains the more corrosion resistant component in a porous "sponge" on the metal surface. It can also occur by redeposition of the noble component of the alloy on the metal surface.
Control is by the use of more resistant alloys-inhibited brasses and malleable or nodular cast iron.

The brass on the left dezincified leaving a porous copper plug on the surface. The gray cast iron water pipe shown on the right photo has graphitized and left graphitic surface plugs which can be seen on the cut surface. The rust tubercules or bubbles are also an indication of pitting corrosion.

The bottom photo shows a layer of copper on the surface of a dealloyed 70% copper-30% nickel cupronickel heat exchanger tube removed from a ship. Stagnant seawater is so corrosive that even this normally corrosion-resistant alloy has corroded. Virtually all copper alloys are subject to dealloying in some environments.