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Historical Articles

September, 1952 issue of Plating

 

HARD CHROMIUM PLATE TO IMPROVE
THE CORROSION RESISTANCE OF TOOL STEEL

H. E. RICKS
Materials Engineering Department, Westinghouse Electric Corporation., East Pittsburgh, Pa.


ABSTRACT
The corrosion protection offered by hard chromium plating was found to be proportional to the thickness of the chromium plate and to the surface smoothness of the panel before plating. For molds, it is recommended that the surface be given a finish of 5 micro inches or less prior to plating and that the chromium plate thickness be at least 0.0005 inch thick.

INTRODUCTION
For some time, controversy has existed as to the value of hard chromium plate in improving the corrosion resistance of tool steels. Hard chromium plated molds are generally accepted by the plastic molding trade as being superior to unplated molds. The hard chromium plate is generally described as very hard wearing and corrosion-resistant.

One of the commonly accepted reasons for hard chromium plating plastic molds is to minimize wear and thus inease the mold-life inasmuch as hard chromium plate has extreme hardness and a low coefficient of friction. In addition, the chromium plate aids in maintaining the desired surface finish on molded parts. Hard chromium plating also serves as a valuable “buildup” tool for salvaging undersized mold parts due to unpredicted growth during heat treatments, mistakes in machining, or normal wear. A major cost reduction consideration in high volume, short cycle molding operations is the excellent anti friction property of chromium plate which practically eliminates down time caused by sticking induced by resin build-up.

The object of the work reported here was to show }ow chromium plate affects the corrosion resistance of tool steel.

EXPERIMENTAL
Panels of tool steel were heat treated to a hardness of Rc 52-55 and were given three different finishes: (1) medium sandblast, 150 micro inches, RMS average; (2) original machined finish 25 micro inches, RMS average; (3) ground and buffed finished 2 micro inches, RMS average.

The panels with the different surface finishes were then divided into four groups; one group was left bare and the other three groups were plated with different thicknesses of chromium, namely: flash, 0.0005 inch and 0.001 inch. The following plating cycle was used:

1. Vapor degrease
2. Immersion soak in hot alkaline cleaner 30 seconds
3. Rinse thoroughly in cold water
4. Reverse etch 30 seconds at 6 volts in plating solution and plate at 2 nmp/sq in (Temperature 131° F)*
5. Rinse thoroughly and air dry

---------------------
*The plating solution analyzed as follows: 27.4 oz/gal trivalent chromium and 0.01 oz/gal iron.

 

TABLE I. TOOL STEEL PANELS AFTER IMMERSION IN CARBONATED DISTILLED WATER
Finish on Panel
Percent of Total Area Rusted
Sand Blasted before Plating (Approx. micro finish
150 RMS)
As Machined before Plating (Approx. micro finish
25 RMS)
Buffed before Plating (Approx. micro finish
2 RMS)
Bare--24 hr immersion
50
20
5
"Flash" chromium plated approx. 0.00005 inch--288 hr immersion
40
10
0
Chromium plated 0.00005 inch--288 hr immersion
2
0
0
Chromium plated 0.001 inch-288 hr immersion
0
0
0
Chromium plated 0.001 inch-361 hr immersion
2
2
0
 


TABLE II. TOOL STEEL PANELS AFTER EXPOSURE TO 95 PERCENT RELATIVE HUMIDITY
Finish on Panel
Percent of Total Area Rusted
Sand Blasted before Plating (Approx. micro finish
150 RMS)
As Machined before Plating (Approx. micro finish
25 RMS)
Buffed before Plating (Approx. micro finish
2 RMS)
Bare--21 hr exposure
95
35
20
"Flash" chromium plated approx. 0.00005 inch--64 hr exposure
8
4
2
Chromium plated 0.00005 inch--161 hr exposure
5
2
1
Chromium plated 0.001 inch-
720 hr exposure
2
1
0
 

 

They were then, immersed in 250 ml pyrex beakers of distilled water through which carbon dioxide was bubbled at the rate of 15 ml per minute. Table I and Fig. 1 show the appearance of the panels after being immersed for different periods of time in the carbonated water.
Another set of panels was exposed to 95 percent relative humidity. Table II and Fig. 2 show the appearance of the panels after exposure to 95 percent relative humidity.

Fig. 1. Tool steel panels after immersion in carbonated water. Fig. 2. Tool steel panels after exposure to 95 percent relative humidity.


DISCUSSION
Figs. 1 and 2 show that chromium plating definitely protects tool steel from carbonated water and from 95 percent relative humidity.
The data were made from visual observation of the panels. Hence, if there appears to be some discrepancy in correlating the tables with the figures, it is due to the difficulty in obtaining realistic photographs.

CONCLUSIONS
1. Hard-chromium plate renders tool steel more corrosion resistant.
2. For a given thickness of chromium plate, greater protection is obtained on a smooth surface than on a rough-surfaced basis metal.
3. For the thicknesses tested, the corrosion protection is directly proportional to the thickness of chromium plate.

RECOMMENDATION
We recommend that mold parts to be hard-chromium plated be given a surface finish of less than 5 micro inches wherever possible before plating and that chromium plate thicknesses of at least 0.0005 inch be used.



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