Bimetallic corrosion of 55% Al -Zn. How they likely to corrode.

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BI-METALLIC CORROSION: RELATED
TO 55%AL-ZN COATED STEEL
SHEETS IN ROOFING AND WALLING
PRODUCTS
Prepared and Presented by: Mya Phu Pwint Thit

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AGENDA
What is Bi-metallic corrosion and galvanic series?
Atmospheric galvanic corrosion in Al-Zn alloys and its coated steel sheets
The corrosion of 55%Al-Zn coated steel sheets in different atmospheres
Why 55%Al-Zn coated steel sheets are optimal to resist corrosion?

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BI-METALLIC CORROSION
• To consider the cause of bi-metallic corrosion, we need to consider the followings:
The galvanic series of materials in specific environment
The environment factors ( e.g., Sea waters influence the galvanic corrosion to spread faster)
The distance factors ( e.g., The corrosion is found to be the most severe in junctions in
materials)
• The passivity, forming oxide layer to prevent further corrosion, influences the galvanic corrosion
behavior.
Figure showing the progress of Bimetallic
corrosion

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Galvanic Series
• The galvanic series of materials is very important
in material selection to prevent the corrosion.
• The farther apart in choosing the two couple
metals from the galvanic series, the greater the
electric potential generated.
• When Al and Zn are coupled,
 The corrosion of Zn is lesser than any other
metals coupling
 Since Al and Zn are in the nearest position
to each other, the electrode potential
generated is less.
 Thus, corrosion rate is expected to be
minimum.

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ATMOSPHERIC GALVANIC CORROSION IN AL-ZN ALLOYS AND ITS COATED STEEL SHEETS
Mechanism
• Zinc is corroded first since it is anodic than aluminum
• When all zinc in interdendritic phase is consumed,
corrosion potential rises to that of aluminum at some
extent of time.
• All the interdendritic phase is filled with zinc and
aluminum corrosion products by the time zinc-rich
portion of coating is corroded gradually.
• These products behave as a passive layer to prevent
further more corrosion.
• The intermetallic layer between the Al-Zn coating and
the steel substrate is more cathodic than steel and
any other components from coating, and therefore
acts as barrier from interdendritic-overlay-corrosion
to the substrate.
The graph showing the corrosion potential of
55% Al-Zn coating in atmosphere Vs.
exposing time

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The graph showing the effect of aluminum content on the corrosion performance of Al-Zn
alloy coatings after 5 years’ exposure in various atmosphere in U.S.A (right)
Why 55%Al-Zn coated steel sheets are optimal to resist
corrosion?

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THE CORROSION OF 55% AL- ZN
COATED STEEL SHEETS IN DIFFERENT
ATMOSPHERES
• These coatings on steel sheets are not suitable to use
for animal confinement buildings because the waste
products generate sulfides and organic acids, which
can corrode the aluminum.
• The coatings cannot be used in alkaline environments
by selectively leaching the aluminum.
• Sulphur is the important corrosion factor in marine
and industrial environments. But formed AlZn sulfate
HYDRATES acts as a protecting barrier.
Galvanize
d
55% Al-Zn coated
SevereMarineIndustrialMarineRural
Figure showing the comparison of the corroded
appearances after 13 years exposing in
atmosphere

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References
[1] “126264795-Corrosion-Engineering-Mars-G-Fontana-McGraw-Hill.pdf.” .
[2] J. J. Friel, “Atmospheric Corrosion Products on Al, Zn, and AIZn Metallic
Coatings,” vol. 42, no. 7, p. 5, 1986.
[3] E. Palma, J. M. Puente, and M. Morcillo, “The atmospheric corrosion
mechanism of 55%Al-Zn coating on steel,” Corros. Sci., vol. 40, no. 1, pp. 61–
68, Jan. 1998.
[4] D. J. Blickwede, “55% Al-Zn-Alloy-Coated Sheet Steel,” Tetsu--Hagane,
vol. 66, no. 7, pp. 821–834, 1980.
[5] “MCA - Service Life Report.pdf.” .
[6] “The Use of Steel Building Panels for Animal Confinement.pdf.” .
http://www.barometal.com/al-zn-coat/