Corrosion

Key Points - Corrosion Resistance

  • All metals used in building construction have a natural inclination to return to the form in which they are found in nature. In the case of aluminum, this natural form is bauxite, or hydrated aluminum oxide.
  • Most types of corrosion (scab corrosion, filoform corrosion, etc.) are addressed by AAMA finishing standards and specifications.
  • Galvanic action will occur when dissimilar metals are in contact while in the presence of an “electrolyte” or conductive fluid. The result of the electrochemical action in the presence of moisture is that the more anodic metal will be corroded.
  • It is necessary to increase the electrical resistance between the two or “turn off the switch.” This can be done by thick elastomeric tapes, non-absorptive plastic, or sealant materials to separate the dissimilar metals. Alternatively, one of the metals, preferably the more cathodic, may be painted or otherwise coated.
  • For aluminum, one coat of chrome phosphate pretreatment followed by primer and high-performance paint will effectively prevent galvanic corrosion. Alternatively, a single field-applied coat of heavy-bodied bituminous paint will usually suffice.
  • The relative areas of the dissimilar metals involved is extremely important. The use of stainless steel screws or rivets (cathodic) in an aluminum window (anodic) is satisfactory and no harmful corrosion is likely to result because the area of the stainless steel is relatively small in relation to the aluminum.
  • A copper roof or fascia located above unprotected aluminum can cause copper salts (cathodic ions) to be deposited on the aluminum by draining rainwater.
  • Carbon steel, whether for reinforcing or steel anchors, when used in conjunction with aluminum window systems, needs to be protected from corrosion and separated against galvanic action. This protection and separation can be accomplished by using zinc or other coatings.
  • Anodic coatings alone are usually insufficient for galvanic separation.