Historical Articles

October, 1952 issue of Plating

 

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This is the type of article for which there has been a considerable demand by our readers. In it is recognized the fact that job shops encounter all sorts of quality in die-castings, and that to overcome this deviation in quality, these shops are forced to vary their plating procedures. Written by a practical man, the paper makes no claims for universal cure-alls, but simply reports the experience of one job shop in licking its problems in the plating of die-castings.

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Copper Striking on Zinc Based Die-Castings

Irvin K. Hauseman, Plating Engineer, Pottstown Plating Works, Inc.

INTRODUCTION
This article, on problems encountered in copper striking, is written to describe how the problems encountered in plating zinc-base die-castings by one job hop were solved and with the thought that this information will prove helpful to other platers. Intensive studies relating to copper striking problems over a six-year period are reported along with different types of faulty plating and their remedies.

REASONS FOR IMPROPER PLATING
It has been found that the cause for a large percentage of plating troubles is improper copper striking. Therefore,- it is well first to check there for trouble when difficulties arise. It is important to have an experienced operator working on the strike tank to assure best results. He must know what to look for; he must know what initial deposit will cover well in follow up plating operations; and he must recognize ‘ a bad looking strike. This operator can check for improper cleaning and rinsing and for water-breaks. His constant reports on how well the zinc base die-castings take a strike and what they look like after striking can be an important factor in quality control.

This constant checking is important in a fully hand operated plating department where manual mistakes in handling cause bad plating even though the system working properly. There cannot be too much inspection or too many precautions taken. Everything may run properly for several hours and then, for some unknown reason, several racks of castings may be blistered or several pieces scratched. It may be that some employee was thinking, for the moment, not of his work but of the date the night before or the fight on TV; or he may have been talking to a fellow worker, and the result is work to strip and to replate.

Trouble, when it comes, can usually be picked up in and around the strike area, so some of the things the operator must watch for and pass judgment on will be discussed.

TYPES OF IMPROPER PLATING
Dullness from Striking
A dull strike simply means one that will not brighten up in subsequent plating. The dullness may be caused by too high a current density, or by the work being in the strike too long, or by the temperature being too low. A dullness that shows here can come from the acid dipping being inadequate to remove the oxide left from the cleaner. If the strike is not high enough in metal content and the color is darkened by overexposure of the work in the strike, the high luster may not come back in the subsequent copper plating operation.
Some jobs may have to be left longer in the strike in order to hide a pattern; this means that a slower strike must be used to keep from burning the work.

Brassy Color from Striking
A brassy color from striking is related to the dull strike in that sometimes the surface becomes sufficiently dull so that a bright copper deposit cannot be obtained. A brassy color may result also from ‘contamination with rather large quantities of zinc. The main cause of this condition has been found to be too light a strike. When the work is transferred to the plating tank it turns a brassy color, sometimes becomes streaked, brightens in patches, and is hard to copper buff. Slight amounts of chrome in the striking solution will also cause this condition which can be corrected by periodic additions of sodium hydrosulfite eg., once every 2 to 3 hours. The chromium contamination could be in the strike or in the copper plating tank and would give the same results. If chromium contamination is very high it can cause blistering, as will be explained under that heading.

Roughness from Striking
Roughness from the strike is particularly bad, because when copper and nickel are plated onto the rough surface the build-up of plating makes the slight roughness a sandpaper surface. Sometimes it becomes so rough that the copper deposit must be buffed and at times it is impossible to eliminate the roughness by buffing. There are times when, coming out of the strike, the slight roughness is hardly visible at a glance, but once in the copper the metal build-up soon shows the roughness on the top side of the work. Therefore, the roughness from the strike must be very closely watched. This is the first thing to check when roughness is encountered. In special cases continuous filtration is used and even then it is difficult to eliminate roughness altogether when operating at capacity. Additions should never be made to plating solutions during working hours because of the probability of causing roughness. When operating at full production, the anodes may darken and smut form on them. Later this smut will float off and collect on top of the solution and cause roughness. Cleaner drag-over from the racks can form a precipitate in the strike which also will cause roughness. It has been found that there is a greater tendency toward roughness when sodium cyanide is used instead of potassium cyanide at a free cyanide value of 1 oz/gal.

Pitting from Striking
Pitting from the strike actually appears as a sandy finish after copper and nickel plating but has no feel of roughness. Sometimes this pitting covers the entire surface of the work and at other times appears to start at the bottom and work up the sides of a casting to form lines or streaks. One cause of this condition is trapped acid in the pores of the metal still present when entering the strike. The cause may be an acid dip which is too strong. This condition may also be corrected by the addition of a wetting agent to the acid dip. Some castings are more porous than others and will show gas bubbles even after two rinses following acid dipping.

A pitting condition can also be caused by contamination of the strike with buffing compounds as well as from wetting, agents being dragged over from the, cleaner. Leaving a casting in the strike too long may cause a fine pitting pattern along the flow line pattern of a casting.

The only real cure is obvious. Better rinsing before the strike is required and at times continuous filtration must be employed.

Casting Pattern from Striking
One of the most difficult problems to solve is the pattern streaking caused by the flow lines of the die castings. On some types of castings, the entire production may have flow lines. Some of the pattern may be covered in plating and in other cases nickel buffing may be required. It is reasonable, then, to believe that if conditions are right none of the castings should require nickel buffing. This condition has been attributed to poor castings, to the cleaner, and to the fact that there may be hard areas in castings that cause patterns to show very plainly after buffing and cleaning.

The fact that the strike seemed to cover the flow lines in some cases and at other times the pattern remained the same as before the piece had the copper strike, it appeared that the answer might be found by determining the proper conditions of striking. After consultation at different times with experts in the field, it was found that with conditions right and the operators running the cleaner and strike properly, all went well.

The conditions developed are as follows:

Instead of running a high metal strike (5.0 to 5.5 oz/gal) using duPont RH-661 salts, the copper content was dropped to 3.5 oz/gal, using copper cyanide. Free sodium cyanide was carried as before 1.0 oz/gal and 2.5 oz/gal of Rochelle Salts were added. The pH was raised from 10.5 to 12.5, and the voltage from 2 to 4 at the generator. Temperature was changed from 140° to 120° F. Time in the strike was changed from 10 to 15 seconds to 30 to 60 seconds. This gave a duller strike that would brighten later in copper plating. This very closely resembles Wagner’s strike solution. 1

It is not good to have a wetting agent in the copper strike in most cases because it tends to produce a bright strike that will not hide the casting: pattern. If the work is not highly buffed a high speed bright strike, with 1 ml/gal of a proprietary wetting agent, can be used that will plate enough copper in to 10 seconds so that the work will not turn dark after transfer to the copper plating tank. But with a bright strike over a highly buffed surface, all the surface defects are reflected more clearly and nothing seems to cover well. For that reason a slow strike without wetting agents is recommended.

Blistering from Striking
Blistering is a common problem to the plater. When blistering is encountered, the work should be checked to see if it has been over-cleaned or over-acid-etched. Blisters probably will not show immediately at the strike, but conditions of cleaning and striking can weaken the bond of the deposit to the base metal so that the plating will blister later either in copper, nickel, or chromium plating, or in baking. 2

High chromium contaminations may cause blistering, as reported by Caldwell. 3 Sodium hydrosulfite may be used to reduce the hexavalent chromium to the trivalent state, but this procedure may not solve the problem entirely. This may be the result of the redissolving and/or re-oxidation of chromium hydroxide as reported by Wagner. 1 Continuous filtration has been found to be helpful in such cases.

The standards of time, concentration and temperature reported have been found satisfactory for one job plating shop. For example, as noted previously, the temperature of the strike was dropped from 140° to 120° F in one special case where the equipment was pushed to its design limits to accomodate a lot of extra large pieces. In transfer of this lot of work on its racks from the strike to the plating tank, it was observed that the die-castings dried prior to completion of the transfer. Solution also dripped from the upper pieces on the dried lower positioned ones and again dried out in the new position. The result was a peeling of copper from copper. Lowering the temperature in this special case was the answer to this problem, and having worked out well in one case, was adopted as the standard operating temperature from that time. Other plants having different set-ups may successfully use different procedures, even though they may be plating the same kind of work.

REFERENCES
1. R.M. Wagner, Plating, 35, 1212-1215 (1948).
2. E.I. Roehl, Metal Finishing, 45, 67 (1947).
3. M.R. Caldwell, Plating, 35, 140 (1948).