Thermal
Spray Metalizing - 85/15 Alloy
Metallines - The Corrosion Bulletin
November 1997
The
85/15 Alloy - Why Bother?
In 1986, Platt Brothers began producing an alloy
of 85% zinc with 15% aluminum wire for the thermal
spray process. It is a difficult alloy to make,
but, after years of study and success in Europe,
Platl believed that this alloy would benefit the
corrosion industry, especially in the fight against
sodium chlorides on our nation's highway bridge
structures.
Why bother with this alloy? The following excerpts
from research done worldwide will attempt to explain
the benefits of 85/15, why the alloy was developed,
the drawbacks of the alloy and information from
equipment manufacturers about the ability to spray
85/15.
The
Development
 85/15
is an alloyed wire of 85% zinc and 15% aluminum
by weight. The alloy was developed in Belgium
by a gentleman by the name of Dunois and the alloy
is so named in some parts of Europe.
This combination and percentage by weight was
derived from development studies of the behavior
of both zinc and aluminum. In 1966 the French
Electricity Research Department published a paper
describing positive behavior of a zinc alloy with
15% aluminum placed for 33 months inside forced
conduits. Further studies of different combinations
of zinc and aluminum revealed that the corrosion
potential of zinc is more negative than that of
iron. The anodic reaction gives sacrificial protection
to steel. Aluminum behaves as an inert coating,
thereby creating a passive type protection to
steel.
After various combinations, the results found
that an increase in the aluminum weight content
of the alloy lead to an increase of aluminum volume.
That is, 28% by weight corresponded to 50% by
volume. At this high content or above, aluminum
became a large volume of the coating. When combined
with zinc, the zinc was not able to provide the
coating with a cathodic protection effect, but
was still sufficient enough to avoid the problem
of underrusting for a short period of time. However,
the zinc s sacrif cial protection was not strong
enough to hinder the corrosion
of the steel in areas of defects, thus behaving
as a pure aluminum coating. Simply
put, any coating using more than 28% by weight
of aluminum acts as a pure aluminum coating.
 Examination
of cross sections of the 85/15 coating lead to
the discovery of a two phase, evenly distributed
coating structure consisting of large elongated
particles containing mainly aluminum surrounded
by tiny particles of zinc. From a structural standpoint,
the coating behaves as a zinc coating which would
be reinforced by a weaving of the more inert aluminum.
Also, after electrochemical analysis, it was concluded
that the two phases combine the fundamental electrochemical
properties of each of the two components.
In 1965, CEFRACOR, the French Anti-Corrosion Centre
carried out a 10 year test on 85/15 in conjunction
with different types of paint coatings in both
atmospheric and immersion testing. Their conclusions
have been documented to these points:
| • |
85/15
has very fine pores and therefore, rapid natural
sealing. |
| • |
High
electrochemical potential and hence constant
sacrificial capability comparable to that
of zinc. |
| • |
An
aluminum base which slows down zinc dissolution
and increases protection duration. |
| • |
From
both technical and aesthetic standpoints,
85/15 can be left unsealed, used in smaller
thicknesses than those required for zinc and
very much smaller than those required for
aluminum. |
| • |
Subsequent
tests have shown that 85/15 has excellent
corrosion performance in chloride or sulfur
dioxide environments both atmospheric and
in underground corrosion applications. |
Platt
Brothers began making production runs of the 85/15
alloy in 1986 for the North American market.
Continued>
E-mail:
thermalspraying@plattbros.com
|
