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CLINIC PLATING pfonline.com/experts Cyanide Zinc on Cast Iron Q. I use a high-cyanide zinc plating bath for plating steel parts, both in barrel and rack lines. I have an opportunity to plate cast iron parts for one of my customers. What would be the correct process for plating zinc on these parts?—G.R. A. The porosity of cast iron tends to make any type of plating more difficult. This doesn't mean that the base material cannot be plated, however. Here is a classic procedure that will give you decent results: Procedure degrease. comments Anodically electroclean. temperature 90oc current density 65-70 asf time 1-2 minutes rinse thoroughly. etch anodically in etch bath. rinse thoroughly. smutted parts must be desmutted. (If etched properly, surface has a gray appearance and no smut.) rinse thoroughly. strike in a stannate tin bath. (A cyanide copper bath at 15 asf for 3 min. can be used instead.) rinse thoroughly. dip in a 30 – 35 g/L sodium cyanide solution. Zinc plate. rinse thoroughly. sulfuric acid Water temperature current density time 205 mL 795 mL 25–30oc 100-250 asf 0.5–1.0 min. sodium cyanide 40 g sodium hydroxide 40 g Water to 1L temperature room current density 70 asf time 1–2 min. Parts are anodic. current density 50 asf time 0.5–1 min. current density 20 asf This is not a simple process, and some experimentation is necessary. Unless there is a large amount of business for you in doing this, I would decline taking the job. One of our readers may have a simpler process to suggest. Yellow Stains on Nickel Plate Q. Yellow stains form in low-current-density areas of the mild steel furniture hinges we nickel barrel plate. The Hull cell test is satisfactory, and I have dummied the plating bath. The current density is 190 amps at 13 V, and I am maintaining pH between 4.2 to 4.6. The barrel rotations are 14 per min. My pre-clean process is degreasing, rinsing, anodic cleaning for 15 min in the barrel at 50°, rinsing with water, sulfuric acid etching and rinsing, then I start plating in the barrel for 1hr and 15 min. The post-cleaning process involves two 30 DECEMBER 2013 — pfonline.com water rinses, a trivalent chrome dip for 2 min, rinsing and drying.—P.D. A. Your plating specifications appear to be in good order. Do you see the yellow stains before you place the nickel-plated parts in the chrome plating bath? How many parts do you plate in your barrel at one time? I'm inclined to think that you are overloading your barrel and not getting adequate plating in the lowercurrent-density regions of your hinges. I suggest that you reduce your barrel load and see if this helps with your problem. Also, you ARTHUR S. KUSHNER could investigate your Consultant rinsing process after the platingclinic@pfonline.com nickel plate to determine if additional rinsing gives you better results. I have sent you a copy of the "Nickel Tank Doctor." Coloring of Stainless Steel Redux Q. Can stainless steel be colored and, if so, can you suggest a procedure for doing so?—Y.P. A. Yes, you can color stainless steel. I addressed the same question in the September 2012 edition of Plating Clinic. Here is what I wrote at that time: Yes, you can color stainless steel, but it is not a process that is typically performed in a plating shop. The basic process, which was developed by Inco (International Nickel) in the 1970s, is to immerse the stainless steel in a chromic and sulfuric acid solution at a temperature just below the boiling point of the solution. A thin layer of chrome oxide forms on the stainless steel, and by controlling the thickness of the oxide layer you can obtain colors that vary from bronze to blue, gold, purple and green. After treatment in this solution, the stainless steel is rinsed and given a cathodic treatment to harden the oxide layer. The layer can vary from 0.02 µm to 0.36 µm; the thicker the layer the darker the color. Most commonly, 304 grade stainless steel is used. The coating is reasonably robust but will scratch. I am aware of one company that produces colored stainless steel, and you should be able to find others as well. Search the PFonline.com database using the term "coloring stainless steel" to find an article about another recently developed process. Some artists use a lacquer-based process for coloring stainless steel which is good for decorative purposes but will not stand up to heavy use.