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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. 

 

Explanation : 

(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.


(2) Classification 

 

The classification letters shown in the
tables indicate the type of deposit to be provided. The types of nickel are
designated: 

b - for nickel deposited in the fully
bright condition.
 

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
deposit).
 


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
chromium.
 

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.
 

TABLE I: RECOMMENDED STANDARDS FOR QUALITY
NICKEL/CHROMIUM FINISHES ON STEEL, IRON AND ZINC PRODUCTS
 


       

Minimum in Thickness
Micrometers



  Typical applications 
  Ni  Cr  Ni Cr   
 SC 1 MILD

Exposure indoors in normally warm, dry atmospheres; coating subject to minimum
wear or abrasion.
 b  r  10  0.1               Toaster bodies, rotisseries, waffle makers. oven doors and liners,
interior auto hardware, trim for major appliances, hair driers, fans,
inexpensive utensils, coat & luggage racks, standing ash trays, interior
trash receptacles, inexpensive light fixtures.
 SC 2 MODERATE

Exposure indoors in

places where condensation of moisture may occur: for example, in kitchens and
bathrooms.
 b/p  r/mc/mp  15~20  0.3  Steel & Iron: stove tops, oven liners, home, office and school
furniture, bar stools, golf club shafts.

Zinc Alloys: bathroom accessories, cabinet hardware.
 SC 3 SEVERE

Exposure which is likely to include occasional or frequent wetting by rain, dew or possibly strong cleaners and saline solutions.
 d/p   r/mc/mp  25~40  0.3  Patio, porch and lawn furniture, bicycles, scooters, wagons, hospital
furniture,fixtures and cabinets.
 SC 4 VERY SEVERE

Service conditions which include likely damage from denting,scratching, or
abrasive wear in addition to

corrosive media.
 d   r/mc/mp  30~40  0.3  Minimum requirements for auto bumpers, grilles, hub caps and lower body
trim. Light housings.
 SC 5 EXTENDED - VERY SEVERE

Service conditions which include likely damage from denting, scratching, or
abrasive wear in addition to

exposure to corrosive environments where long time protection of the substrate
is
 d  mc/mp  35  0.3  Extended usage in exterior automotive and other items as in SC 4 above.


Note: 5 micrometers copper is required on zinc and zinc alloys prior to
nickel-chromium. 15 micrometers may be used on steel.

25.4 micrometers = 1.0 mil

Adapted from ASTM Standard B-456

TABLE II: RECOMMENDED STANDARDS FOR
QUALITY NICKEL/CHROMIUM FINISHES ON COPPER AND COPPER ALLOYS

 

 Service Conditions

Classification



 
   

Minimum in Thickness
Micrometers



   Typical Applications
   Ni  Cr  Ni  Cr  
 SC 1 MILD

Exposure indoors in normally warm, dry atmospheres with coating subject to
minimum wear or abrasion.
 b/p  r               5  0. 1                Toaster bodies and similar appliances, oven door and liners, interior
autro hardware, trim for major appliances, receptacles, and light fixtures.
 SC 2 MODERATE

Exposure indoors in places where condensation of moisture may occur: for
example, in kitchens and bathrooms.
 b/p  r/mc/mp  10~15  0.3  Plumbing fixtures, bathroom accessories, hinges, light fixtures,
flashlights, and spot lights.
 SC 3 SEVERE

Exposure which is likely to include occasional or frequent wetting by rain or
dew or possibly strong cleaner and saline solutions.
 d/b/p  r/mc/mp  20~30  0.3  Patio, porch and lawn furniture and light fixtures, bicycle parts,
hospital and laboratory fixtures.
 SC 4 VERY SEVERE

Service conditions which include likely damage from denting, scratching or
abrasive wear in addition to corrosive media
 d  r/mc/mp  25~30  0.3  Boat fittings, auto trim, hub caps, lower body trim.

 

 


 
 
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