The Voice of the Finishing Industry since 1936

  • PF Youtube
  • PF Facebook
  • PF Twitter
  • PF LinkedIn
9/1/2014 | 1 MINUTE READ

Zinc Plating on Cast Iron

Facebook Share Icon LinkedIn Share Icon Twitter Share Icon Share by EMail icon Print Icon

Why are cast iron parts coming out of chromating with dark areas?

Q. We are a zinc plating shop with both cyanide and acid chloride plating lines. We are able to do quality work on all of the parts that we receive with the exception of components made of cast iron. When we plate cast iron we end up with dark areas on the surface after the chromating step. Prior to the trivalent chromating step, we do not see the dark areas. What are we doing wrong? –R.R.

A. You don't mention in your message whether you are plating the cast iron parts with cyanide zinc or the acid chloride zinc. Cyanide zinc is not the best choice for plating cast iron. Assuming you are using the acid zinc plating bath, your problem is most likely due to not having enough of a zinc layer on the cast iron parts. When the parts are placed in the chromate conversion bath, some of the zinc is removed from the part during the conversion step. If there isn’t enough zinc to start with, you’ll end up with dark spots in those areas. The solution is straightforward: apply a heavier zinc deposit. You should also discuss this problem with the vendor of your chromium conversion coating.

Related Topics


  • Gold and Silver Plating Basics

    An overview of precious metal electroplating processes.

  • Chromium Plating

    An overview of decorative and hard chromium electroplating processes.

  • Masking for Surface Finishing

    Masking is employed in most any metal finishing operation where only a specifically defined area of the surface of a part must be exposed to a process. Conversely, masking may be employed on a surface where treatment is either not required or must be avoided. This article covers the many aspects of masking for metal finishing, including applications, methods and the various types of masking employed.