49
(a) Nonmetallic Coatings
2
Chemical
Glass-
Polymer
Conversion
Ceramic
Oxide
Anodizing
Phosphate
Chromate
(b) Metallic Coatings
Diffusion
Plating
Hot Dip
Spray
Deposition
Chemical
Mechanical
Electroplating
Electroless
Immersion
Vacuum
Vapor
F I G U R E 2 - 16
Coating Methods Available for Metals
Table 2-4
rings are chrome-plated to improve wear resistance, fasteners are plated to reduce cor-Galvanic Series
rosion, and automobile trim is chrome-plated for appearance and corrosion resistance.
of Metals in Seawater
Figure 2-16 shows a chart of various types of coatings for machine applications. These divide into two major classes, metallic and nonmetallic, based on the type of coating, Least noble
not substrate. Some of the classes divide into many subclasses. We will discuss only Magnesium
a few of these here. The reader is encouraged to seek more information from the references in the bibliography.
Zinc
Aluminum
Cadmium
Galvanic Action
Steel
When a coating of one metal is applied to another dissimilar metal, a galvanic cell may Cast iron
be created. All metals are electrolytically active to a greater or lesser degree and if suf-Stainless steel
ficiently different in their electrolytic potential will create a battery in the presence of Lead
a conductive electrolyte such as seawater or even tap water. Table 2-4 lists some com-Tin
mon metals ordered in terms of their galvanic action potential from the least noble (most Nickel
electrolytically active) to the most noble (least active). Combinations of metals that are Brass
close to each other in the galvanic series, such as cast iron and steel, are relatively safe from galvanic corrosion. Combinations of metals far apart on this scale, such as alu-Copper
minum and copper, will experience severe corrosion in an electrolyte or even in a moist Bronze
environment.
Monel
Silver
In a conductive medium, the two metals become anode and cathode, with the less-Titanium
noble metal acting as the anode. The self-generated electrical current flow causes a loss of material from the anode and a deposition of material on the cathode. The less-noble Graphite
metal gradually disappears. This problem occurs whenever two metals sufficiently far Gold
apart in the galvanic series are present in an electrically conductive medium. Thus, not Platinum
only coatings but fasteners and mating parts must be made of metal combinations that Most noble
will not create this problem.
50
MACHINE DESIGN -
An Integrated Approach
Electroplating
Electroplating involves the deliberate creation of a galvanic cell in which the part to be 2
plated is the cathode and the plating material is the anode. The two metals are placed
in an electrolyte bath and a direct current applied from anode to cathode. Ions of the
plating material are driven to the plating substrate through the electrolyte and cover the
part with a thin coating of the plating material. Allowance must be made for the plat-
ing thickness, which is controllable. Plating thickness is uniform except at sharp cor-
ners or in holes and crevices. The plating builds up on the outside corners and will not
go into holes or narrow crevices. Thus, grinding may be necessary after plating to re-
store dimensions. Worn parts (or mistakes) can sometimes be repaired by plating on a
coating of suitable material, then regrinding to dimension.
Steels, nickel- and copper-based alloys, as well as other metals are readily electro-
platable. Two approaches are possible. If a more noble (less active) metal is plated onto
the substrate, it can reduce the tendency to oxidize as long as the plating remains in-
tact to protect the substrate from the environment. Tin, nickel, and chromium are of-
ten used to electroplate steel for corrosion resistance. Chrome plating also offers an
increase in surface hardness to HRC 70, which is above that obtainable from many hard-
ened alloy steels.* Unfortunately, any disruptions or pits in the plating can provide nodes
for galvanic action if conductive media (such as rainwater) are present. Because the sub-
strate is less noble than the plating, it becomes the sacrificial anode and rapidly corrodes.
Electroplating with metals more noble than the substrate is seldom used for parts that
will be immersed in water or other electrolytes.
Alternatively, a less-noble metal can be plated onto the substrate to serve as a sac-
rificial anode which will corrode instead of the substrate. The most common example
of this is zinc coating of steel, also called galvanizing. (Cadmium can be used instead
of zinc and will last longer in saltwater or salt-air environments.) The zinc or cadmium
coating will gradually corrode and protect the more noble steel substrate until the coating
is used up, after which the steel will oxidize. Zinc coating can be applied by a process
called “hot dipping” rather than by electroplating, which will result in a thicker and more
protective coating recognizable by its “mother-of-pearl” appearance. Galvanizing is of-
ten applied by manufacturers to automobile body panels to inhibit corrosion. Sacrifi-
cial zinc anodes are also attached to aluminum outboard motors and aluminum boat hulls
to short-circuit corrosion of the aluminum in seawater.
A caution about electroplated coatings is that hydrogen embrittlement of the substrate can occur, causing significant loss of strength. Electroplated finishes should not
be used on parts that are fatigue loaded. Experience has shown that electroplating se-
* It is interesting to note that
verely reduces the fatigue strength of metals and can cause early failure.
chromium in the pure form is
softer than hardened steel but
when electroplated onto steel, it
becomes harder than the steel
Electroless Plating
substrate. Nickel and iron also
increase their hardness when
Electroless plating puts a coating of nickel on the substrate without any electric current electroplated on metal substrates.
The mechanism is not well
needed. The substrate “cathode” in this case (there is no anode) acts as a catalyst to start understood, but it is believed that
a chemical reaction that causes nickel ions in the electrolyte solution to be reduced and internal microstrains are developed
in the plating process that harden
deposited on the substrate. The nickel coating also acts as a catalyst and keeps the re-the coating. The hardness of the
plating can be controlled by
action going until the part is removed from the bath. Thus, relatively thick coatings can changes in process conditions.
be developed. Coatings are typically between 0.001 in and 0.002 in thick. Unlike elec-