Electrodeposition
Nickel, chromium and zinc are commonly used as electrodeposits.
Chromium, the hardest of these coatings, is applied for abrasion
resistance
Properties and Selection of Materials 99
and low coefficient of friction. Nickel and zinc electrodeposits are
used for resistance to corrosion, the latter for mildly corrosive
conditions [39,40].
Dip Coating
Dip coating involves immersion of steel or copper in a bath of molten
coating metal (zinc, tin and/or aluminum). Hot dip-galvanized
(zinc-coated) steel should not be used in circuits containing copper
equipment. This can result in galvanic corrosion at the
copper/galvanized junctions, as well as cause overall galvanic
corrosion of the zinc by copper redepositing from the water or process
stream. Galvanized equipment is not recommended for use with liquors
above 60°C. Above this temperature there is a reversal of the polarity
of the zinc/steel couple, and the coating ceases to be protective where
flaws appear in the coating. Impervious coatings of tin for mild
corrosive conditions can be formed on steel and copper by dipping in a
molten bath of tin.
Aluminum is the highest melting point metal (660°C) applied by hot
dipping. Aluminized steel can be used at temperatures up to 550°C
without appreciable oxidation. This steel has very good resistance to
gases and vapors containing small quantities of sulfur dioxide and
hydrogen sulfide .
Sprayed Coatings
Zinc, aluminum, nickel alloys, cobalt alloys and tungsten carbide are
applied for sprayed coatings, which are slightly porous. Flame-sprayed
zinc coatings are used for corrosion protection of steel and provide
similar properties for galvanized coatings.
Sprayed aluminum coatings used on steel for protection against
atmospheric corrosion are preferred over zinc for use in areas with
considerable contamination of the atmosphere by sulfur oxides [44].
Sprayed aluminum also is used for the protection of steel at elevated
temperatures up to 550°C. For temperatures of 550-900°C, aluminum is
converted to a high-melting point aluminum/iron compound by heating the
coated equipment to 800/900°C and maintaining it at that temperature
for 15 minutes. For protection up to 1000°C, a sprayed coating of
nickel chromium and nickel and cobalt alloys is applied. Nickel or
cobalt alloys containing small amounts of boron or silicon can be
deposited with very simple equipment, requiring very little heating of
the base metal.
Diffusion Coatings
The purpose of diffusion coatings is not to produce a coating of
another metal on the substrate, but to change the composition of the
surface layers of the substrate by alloying with the diffusing metal chosen (zinc,
aluminum, chromium and silicon). The surface properties after such
treatment depend not only on the metal diffused, but also on the
composition of the substrate. The diffusion coating causes very little
change in the dimensions of the piece being treated, which is important
for items machined to fine limits, such as nuts and bolts [45,46].
Zinc diffusion is used for protection against atmospheric corrosion.
Aluminum diffusion is used to improve the oxidation resistance of
low-carbon steels.
Chromium diffusion applied to a low-carbon steel produces a surface
that has the characteristics of ferritic stainless steel, such as
AISI446 to a depth about 0.1 mm. When diffusion is applied to a
high-carbon steel, a surface rich in chromium carbides is formed. This
has a hardness greater than 1000 VHN, which provides good resistance to
abrasion.
Nickel alloys and stainless steels such as AISI310 (25Cr/20Ni)
diffusion treated with chromium enhance resistance to sulfur gases at
high temperatures. The chromium-rich surface prevents the formation of
nickel sulfide.
The use of equipment close to the temperature at which the material was
diffusion treated will result in continuing diffusion of chromium,
aluminum etc., into the substrate, thus depleting chromium with
consequent loss in oxidation and corrosion resistance. For aluminum,
this effect is noticeable above 700°C in steels, and above 900°C in
nickel alloys. For chromium, the effect is pronounced above 850°C for
steels and above 950°C for nickel alloys.
Silicon used for diffusion treatment of carbon steels enhances
corrosion resistance to sulfuric acid. Such a treatment has the surface
durability of iron/silicon alloys without their marked brittleness.