This document discusses bi-metallic corrosion that can occur with 55% aluminum-zinc coated steel sheets used in roofing and walling products. It explains that aluminum-zinc coatings are optimal for resisting corrosion when used in atmospheric conditions because aluminum and zinc are adjacent in the galvanic series, generating minimal electrode potential and corrosion rate. The coating protects the steel substrate as the zinc corrodes first, then forms a passive layer of corrosion products. However, the coating is not suitable for use in environments containing sulfides, organic acids, or alkaline conditions, as these can corrode the aluminum.
1 of 9
More Related Content
Bimetallic corrosion of 55 alzn
1. 1
BI-METALLIC CORROSION: RELATED
TO 55%AL-ZN COATED STEEL
SHEETS IN ROOFING AND WALLING
PRODUCTS
Prepared and Presented by: Mya Phu Pwint Thit
2. 2
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?
3. 3
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
4. 4
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.
5. 5
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
6. 6
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?
7. 7
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
9. 9
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. 821834, 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/
Editor's Notes
#2: Welcome all Ls & Gm
Good Afternoon!
The topic I would like to share my knowledge with u is