65    EVALUATION OF NONCYANIDE METAL STRIPPERS
Stuart K. Janikowski, Scientist
Peter A. Pryfogle, Senior Scientist
Idaho National Engineering Laboratory
Idaho Falls, Idaho 83415
INTRODUCTION
The cyanide ion has many chemical properties that make it suitable for use in the metal finishing
and refurbishing industry. It lowers the reduction potential of many metals' making them more
susceptible to oxidation under relatively mild conditions, and hence, enhances metal stripping for
commonly plated materials.2 It is a strong chelating agent3 that serves to clean metal surfaces of
oxidized species in cleaning and stripping applications, or hold high concentrations of metal ions in
electroplating solutions. Cyanide also participates in several reversible electrochemical reactions that
make it suitable for electroplating certain metals.2 Because of these characteristics, concentrated
cyanide solutions are widely used in the electroplating industry: 1) in cleaning formulations to remove
scale, smut, rust, and other surface contaminants before electroplating new parts or refurbishing used
parts; 2) in electroplating solutions for zinc, cadmium, copper, silver, and gold; and 3) for plate metal
stripping prior to refurbishing used parts.4
In today's atmosphere of worker awareness, environmental concern, and long term liability, the
hazards of continued cyanide use are becoming unacceptable. The spent solutions containing cyanide
are considered hazardous wastes and are a significant fraction of the total wastes generated at
electroplating facilities.5 In most industrial waste treatment plants, isolated transport lines and chemical processing facilities are required to destroy the cyanide prior to metals precipitation and biological
degradation of the remaining constituents. If acceptable noncyanide formulations can be substituted
for the cyanide solutions, the magnitude of these disadvantages can be greatly diminished.
The purpose of the work reported here was to test and evaluate commercially available noncyanide
metal stripping products. Three types of tests were conducted in the laboratory to evaluate the
stripping performance, basis metal protection, and biodegradability characteristics of the solutions.
The tests were conducted using some parameters specific to and reflecting on conditions commonly
encountered at U.S. Air Force installations.
EXPERIMENTAL
Stripping tests were conducted in borosilicate glass reactors roughly 2 1/4 inches in diameter by 8
inches tall. A schematic of the reactor assembly is shown in Figure 1. Metal coupons were suspended
from support rods in a nominal volume of 180 mL of the stripper solution made to the manufacturer's
specifications. Two coupons were suspended in each of the reactors, one of the test samples and the
other a "dummy" coupon of equal surface area. Test samples were either plate or basis metals 1 x 2
x 1/8 or 1/16 inch in size and were sanded to a 120 grit surface. The dummy coupons used opposite
the plate and basis metals were stainless steel or passivated stainless steel, respectively. Electrical
connections were made between the supports to simulate galvanic effects that would be present under
immersion operating conditions in a job shop. Under electrolytic operating conditions, the supports
were connected to a d.c. regulated power supply and stainless steel cathodes were used. Agitation of
the solution was supplied by air sparge or magnetic stirring, as per the manufacturer's recommendations. The coupons were cleaned6, tared7, and immersed8 in the stripper solution, after which the
reactor was immersed in a constant temperature water bath, maintained at the desired temperature, to
a height just above the solution level. The operating temperatures were the highest recommended or to
a maximum of 130°F. At designated time intervals, the reactors were removed from the bath and the
coupons were rinsed with deionized water and immersed in acetone, wiped dry, further dried at 50°C
in an oven, and then placed in a desiccator to cool.6 Duplicate tests were run for each sample type.
The times ranged from one half to 6 and 1 to 24 hours for the plate and basis metals, respectively. The
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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