5. Corrosion form under atmospheric environment
5.1.Flat part
Corrosion forms under atmospheric environment of W.R.Alstar, Type 2 and 55% Al-Zn are discussed below:

(1) W.R.Alstar
Photo-8 shows corrosion forms under atmospheric exposure tests, from which the following observations can be made.
  • Corrosion of aluminum layer starts around eutectic silicon and spreads along silicon structure. Silicon itself corrodes little. (a)
  • Corrosion is progressed by three-dimensional network of eutectic silicon and reaches the alloy layer. (b)
  • Thus spotty corrosion of aluminum layer visible with naked eyes develops but under normal environment, this corrosion rarely reaches the substrate; it only extends horizontally. (c)
  • Corrosion of the substrate is prevented as long as there exists metallic aluminum.

Photo-8. Corrosion form of W.R.Alstar under atmospheric exposure tests (X500)

(2) ASTM A463 Type 2
Photo-9 shows corrosion form under atmospheric exposure tests, from which the following observations can be made.
  • In case of pure aluminum type, corrosion of aluminum layer starts around and extends along the needle-like intermetallic composite. Intermetallic composite itself corrodes little. (a)
  • Corrosion is progressed by three-dimensional network of intermetallic composite and reaches the alloy layer. (b)
  • Thus spotty corrosion of aluminum layer visible with naked eyes develops but under normal environment, this corrosion rarely reaches the substrate; it only extends horizontally. (c)
  • Corrosion of the substrate is prevented as long as there exists metallic aluminum.

Photo-9. Corrosion form of Type 2 (pure aluminum type) under atmospheric exposure tests (X500)

(3) 55% Al-Zn
In the coating layer of 55% Al-Zn both Al-rich phase and Zn-rich phase are intermixed and corrosion of the coating layer under atmospheric exposure tests progresses as shown in Photo-10.
  • Zn-rich phase is selectively corroded and corrosive growth of zinc is piled up inside and on the surface of the coating layer. (a)
  • As this zinc growth is water-dissolved, pH at the boundary of Éø-Al phase and corroded Zn-rich phase in the coating layer is substantially lowered. Consequently, Al starts to corrode in the coating layer and sulfuric acid aluminum develops, which with further water-dissolution, turns base sulfuric acid aluminum. Thus corrosion of aluminum in the coating layer starts where it contacts with Zn-rich phase in the layer.
    When such partial corrosion reaches the boundary of the coating layer and alloy layer, corrosion progresses along the boundary and with the development of corrosive growth, pushes up the coating layer. (b)
  • Further progress of corrosion, along with the development of corrosive growth, pushes up the remaining coating layer. (c)

Photo-10. Form of corrosion of 55% Al-Zn under atmospheric exposure tests



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