Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

Issue link:

Contents of this Issue


Page 347 of 707

ness, it is impossible to meet both requirements with a single solution. Brush plating manufacturers are keeping up with the movement toward green solutions, offering alternatives to cadmium, such as zinc-nickel and tin-zinc. Additionally they are offering trivalent chromium conversion coatings for zinc- nickel, and anodized coatings. ADHESION: The adhesion of brush electroplates is excellent and comparable to that of good tank plating on a wide variety of materials including steel, cast iron, stainless steel, copper, high temperature nickel-base materials, etc. When plating on these materials, the adhesion requirements of federal and military specifications are eas- ily met. Limited, but occasionally useful, adhesion is obtained on metals that are difficult to plate such as tungsten, and tantalum. There is now a process avail- able for achieving very good adhesion on common titanium alloys. Most adhesion evaluations have been made using destructive qualitative tests such as chisel or bend tests. These tests indicate that the adhesion and cohesion of brush plated deposits is about the same as the cohesive strength of the base material. Quantitative tests have been run using ASTM Test Procedure C-653-79 "Standard Test Method for Adhesion or Cohesive Strength of Flame Sprayed Coating." As an example four samples were plated using a nickel neutral solution. The cement used to bond the nickel plated sample to the testing apparatus failed during the test. Since the adhesive had a bond strength rated at approx- imately 11,300 psi, it was shown that the bond strength of the plated deposit to the substrate is at least 11,300 psi. Even brush plated deposits with a fair adhe- sive rating survived this test and, therefore, have an adhesive bond and cohesive strength of at least 11,300 psi. Therefore, brush plated bonds are stronger than the bonds found with flame sprayed coatings. METALLOGRAPHIC STRUCTURE: The metallographic structure of an electroplate can be examined in an etched or unetched condition. In the unetched condition, most brush plated deposits are metallurgically dense and free of defects. Some of the harder deposits, such as chromium, cobalt-tungsten, and the hardest nickel, are microcracked much like hard tank chromium. A few deposits are deliberately microporous, such as some of the cadmium and zinc deposits. Microporosity does not affect the corrosion protection of these deposits, since they are intended to be sacrificial coatings. The microporous structure offers an advantage over a dense deposit because it permits hydrogen to be released out naturally at ambient temperatures or in a baking operation. Etched brush plated deposits show grain structures that vary, but parallel those of tank deposits. However, brush plated deposits tend to be more fine grained. Coarse grained, columnar structures, such as those found in Watts nickel tank deposits, have not been seen in brush plated deposits. HARDNESS: The hardness of brush plated deposits lies within the broad range of the hardnesses obtained with tank deposits. Brush plated cobalt and gold, however, are harder than tank 346

Articles in this issue

view archives of Metal Finishing Guide Book - 2011-2012 Surface Finishing Guidebook