How to specify the Chrome plating for variable parts?
SPECIFYING THE FINISH
Having selected copper/nickel/chromium or
nickel/chromium as the coating system for a steel, plastic, stainless steel,
brass, zinc or other substrate, the designer must now specify the type,
thickness and other characteristics desired in the coating. High quality can be
obtained by properly specifying the types and thickness of layers to be
applied. The designer must specify the appearance of the layer's finished
surface. These will determine the protective value of the final part. The
required performance tests such as adhesion and ductility should also be
specified. For specific subjects, refer to the 1991 Annual Book of ASTM
Standards, Volume 02.05, which covers Electrodeposited Coatings.
Type and Thickness
By type is meant the number and sequence of layers of copper, nickel and
chromium which constitute the coating. A variety of multiple-layer decorative
finishes have been developed by the various suppliers. Most are superior in
protective value to a single layer of bright nickel covered by chromium. The
large number of deposit types and combinations of layers can economically meet
most performance specifications.
(1) Service Conditions
Depending upon the customer, there are
four or five service conditions which define the environment to which the
plated part may be exposed as a function of the substrate.
The classification letters shown in the
tables indicate the type of deposit to be provided. The types of nickel are
b - for nickel deposited in the fully
p - for dull or semi-bright nickel
requiring polishing/buffing to give full brightness, and containing less than
0.005 mass % sulfur (Note 0.005 mass % of sulfur is essentially a sulfur-free
d - for a double-layer or triple-layer
nickel coating of which the bottom layer contains less than 0.005 mass % sulfur
and the top layer contains more than 0.04 mass % sulfur. The low sulfur layer
should be from 60% to 75% of the total nickel thickness. If there are three
layers, the intermediate one shall contain not less than 0.15 mass % sulfur and
shall not exceed 10% of the total nickel thickness.
There is no restriction on the type of
chromium used, except that one is not permitted to buff the chromium deposit.
There are no restrictions on how the microporous and microcracked deposits are
produced. The deposits must meet the following classifications:
r - for regular (i.e., conventional)
chromium. This deposit is non-microdiscontinuous hexavalent or trivalent
Mc - for microcracked chromium having more
than 300 cracks per linear cmi (750/in.) in any direction over significant
surfaces. The cracks shall be invisible to the unaided eye.
mp - for microporous chromium containing a
minimum of 10,000 pores per sq. cm. ( 65,000/sq.in.). The pores shall be
invisible to the unaided eye.