Chapter 2

MATERIALS AND PROCESSES

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.

 

Image 146

 

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-

 

Image 147