How to Avoid Contact With Aluminum & Dissimilar Metals

This is provided to inform clients of the dissimilar metal protection requirements of the 2015 Aluminum Design Manual, Chapter M – Section M.7

Structures of the alloys covered by the Aluminum Design Manual are not ordinarily painted. Surfaces shall be painted where:

• 2014-T6 alloys are exposed to corrosive environments.
• Aluminum alloy parts are in contact with, or are fastened to, uncoated steel members or other dissimilar materials (see below).
• Exposed to extremely corrosive conditions.
• Required by the designer for reason of appearance.

Either the aluminum or the dissimilar material must be painted, sealed, or otherwise protected from contact with the following materials:

• • STEEL
    • Exceptions: No protection required for:
      • Stainless Steel (300 series), except in high-chloride (salty) environments.
      • Galvanized Steel (hot-dip galvanized or electro-galvanized).
      • Aluminized steel.
    • Acceptable Paints:
      • Priming Paint (one coat), such as zinc molybdate, alkyd type primer in accordance with Federal Specification TT-P-645B (“good quality”, no lead content).
      • Follow primer with two coats of paint consisting of 2 lb. of aluminum paste pigment (ASTM Specification D962-81, Type 2, Class B) per gallon of varnish meeting Federal Specification TT-V-8 1, Type 11, or equivalent.
      • Severe Corrosion Conditions: Additional protection can be obtained by applying a suitable sealant to the faying surfaces, capable of excluding moisture from the joint during prolonged service in addition to the zinc molybdate, alkyd type primer.

     

  • WOOD
    • Includes aluminum laid on or against:
      • Wood
      • Fiberboard
      • Other porous materials that absorb water and cause corrosion (i.e. where moisture is present and corrodents will be entrapped between the surfaces).
    • Acceptable Protection:
      • Heavy coat of alkali resistant bituminous paint or other coating providing equivalent protection before installation.
      • Other insulating barrier between the aluminum and the porous material.

• • CONCRETE, Embedded into
    • Exceptions:
      • Concrete with no corrosive additives – no protection required.
      • Where no extended exposure to extremely corrosive conditions is expected – no protection required.
    • Acceptable Paints:
      • Priming Paint (one coat), such as zinc molybdate primer in accordance with Federal Specification TT-P-645B (“good quality”, no lead content).
      • Alt: Heavy coating of alkali resistant bituminous paint.
      • Alt: Wrap aluminum with a suitable plastic tape applied in such a manner as to provide adequate protection at the overlaps.
    • NOTE: Aluminum shall not be embedded in concrete to which corrosive components such as chlorides have been added if the aluminum will be electrically connected to steel.

• • • WATER that has come in contact with a heavy metal (e.g. copper)
      • Heavy metals leached into water can cause corrosion of aluminum.
      • Acceptable Remedies:
        • Paint the heavy metal.
        • Coat the heavy metal with plastic.
        • Divert drainage from heavy metal away from aluminum.

Prepainted aluminum only requires additional protection when specified by the designer to abate extremely corrosive conditions.

Information provided by Zach Rubin, PE