Corrosion of Metallic Coatings
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The finish is continuous and very uniform, even on threaded and irregular parts. This is a very useful finish for nuts and bolts, which, with proper
allowance for the thickness of coats, can be sherardized after manufacture
and used without retapping the threads.
The thickness of the coating can be controlled. Usually, a thickness of 30 μm
is used for outdoor applications while 15 μm is used for indoor applications.
Thermally sprayed coatings. In this process, droplets of semimolten zinc are
sprayed from a special gun that is fed with either wire or powder onto a gritblasted surface. The semimolten droplets coalesce with some zinc oxide present at each interface between droplets. Electrical continuity is maintained
both throughout the coating and with the iron substrate so that full cathodic
protection can be obtained because the zinc oxide forms only a small percentage of the coating.
The sprayed coating contains voids (typically 10 to 20% by volume) between
coalesced particles. These voids have little effect on the corrosion protection
because they soon fill up with zinc corrosion products and are thereafter
impermeable. However, the use of a sealer to fill the voids improves appearance in service and adds to life expectancy but, more importantly, it provides a better surface for subsequent application of paint.
There are no size or shape limitations regarding the use of this process.
Zinc Dust Painting. Zinc dust paints can be used alone for protection or as
a primer followed by conventional topcoats.
8.4.1.2 Corrosion of Zinc Coatings
In general, zinc coatings corrode in a similar manner as solid zinc. However,
there are some differences. For example, the iron-zinc alloy present in most
galvanized coatings has a higher corrosion resistance than solid zinc in neutral and acid solutions. At points where the zinc coating is defective, the bare
steel is cathodically protected under most conditions.
The corrosion of zinc coatings in air is an approximate straight-line relationship between weight loss and time. Because the protective film on zinc
increases with time in rural and marine atmospheres of some types, under
these conditions the life of the zinc may increase more than in proportion to
thickness. However, this does not always happen.
Zinc coatings are used primarily to protect ferrous parts against atmospheric corrosion. These coatings have good resistance to abrasion by solid
pollutants in the atmosphere. General points to consider include:
1. Corrosion increases with time of wetness.
2. The corrosion rate increases with an increase in the amount of sulfur compounds in the atmosphere. Chlorides and nitrogen oxides
usually have a lesser effect but are often very significant in combination with sulfates.
283
The finish is continuous and very uniform, even on threaded and irregular parts. This is a very useful finish for nuts and bolts, which, with proper
allowance for the thickness of coats, can be sherardized after manufacture
and used without retapping the threads.
The thickness of the coating can be controlled. Usually, a thickness of 30 μm
is used for outdoor applications while 15 μm is used for indoor applications.
Thermally sprayed coatings. In this process, droplets of semimolten zinc are
sprayed from a special gun that is fed with either wire or powder onto a gritblasted surface. The semimolten droplets coalesce with some zinc oxide present at each interface between droplets. Electrical continuity is maintained
both throughout the coating and with the iron substrate so that full cathodic
protection can be obtained because the zinc oxide forms only a small percentage of the coating.
The sprayed coating contains voids (typically 10 to 20% by volume) between
coalesced particles. These voids have little effect on the corrosion protection
because they soon fill up with zinc corrosion products and are thereafter
impermeable. However, the use of a sealer to fill the voids improves appearance in service and adds to life expectancy but, more importantly, it provides a better surface for subsequent application of paint.
There are no size or shape limitations regarding the use of this process.
Zinc Dust Painting. Zinc dust paints can be used alone for protection or as
a primer followed by conventional topcoats.
8.4.1.2 Corrosion of Zinc Coatings
In general, zinc coatings corrode in a similar manner as solid zinc. However,
there are some differences. For example, the iron-zinc alloy present in most
galvanized coatings has a higher corrosion resistance than solid zinc in neutral and acid solutions. At points where the zinc coating is defective, the bare
steel is cathodically protected under most conditions.
The corrosion of zinc coatings in air is an approximate straight-line relationship between weight loss and time. Because the protective film on zinc
increases with time in rural and marine atmospheres of some types, under
these conditions the life of the zinc may increase more than in proportion to
thickness. However, this does not always happen.
Zinc coatings are used primarily to protect ferrous parts against atmospheric corrosion. These coatings have good resistance to abrasion by solid
pollutants in the atmosphere. General points to consider include:
1. Corrosion increases with time of wetness.
2. The corrosion rate increases with an increase in the amount of sulfur compounds in the atmosphere. Chlorides and nitrogen oxides
usually have a lesser effect but are often very significant in combination with sulfates.