Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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directly on the plating solution to remove all common metallic impurities. This eliminates the build up of metallic impurities and excludes any change in deposit color or properties associated with metallic impurities. If a trivalent chromium plating solution has to be waste treated its cost is approximately one-tenth that of treating an equal volume hexavalent chromium. Hexavalent chromium processes are essentially insensitive to organic conta- mination since the hexavalent chromium ion destroys most organics, resulting in the formation of trivalent chromium ions. Being a contaminant, an excess of trivalent chromium must be reconverted back to hexavalent chromium. The com- mon way is to dummy at a high cathode current density (e.g., anode current den- sity of 12 A/ft2, cathode current density of 600 A/ft2). Trivalent chromium processes are also relatively insensitive to organic impurities but sometimes organics must be removed. Occasional carbon filtering is sufficient for some processes while routine carbon/peroxide treatments are needed for others. CORROSION PROTECTION Decorative chromium deposits play an important role in the base metal protection provided by nickel/chromium systems. They offer hardness, appealing color, tar- nish resistance, wear resistance, and corrosion resistance. Even though decora- tive trivalent and hexavalent chromium deposits are used interchangeably, there are some important differences. For example, hexavalent chromium ions impart short-term corrosion protection on non-chromium plated surfaces by "chro- mating" the part. Trivalent ions do not and so post-plating treatments are nec- essary to obtain the equivalent protection When corrosion resistance is important, most specifications encourage or require micro-discontinuous chromium deposits. With a controlled pattern of microscopic pores or cracks, the corrosion potential between the chromium and underlying nickel deposits is spread out over thousands of corrosion sites. This reduces the anodic current on the nickel at any one site thus greatly reduc- ing the individual corrosion rate. This results in a fine pattern of corrosion sites (Active Sites) uniformly spaced over the surface. A typical standard will spec- ify a minimum of 10,000 micropores per square centimetre or over 30 microcracks per millimetre. Without micro-discontinuity all the corrosion potential is con- centrated in a few sites resulting in unsightly, irregularly spaced, large corrosion sites. Hexavalent chromium deposits must undergo special treatments to produce micro-discontinuity. Plating chromium over very fine inert particles that are code- posited in a nickel strike (particle nickel) over the bright nickel deposit is the typ- ical way of producing microporous chromium. Lightly spraying the hard, brittle chromium deposit with hard 60 to 80 mesh particles produces microporous chromium at the contact points. Some trivalent chromium deposits are micro- discontinuous as plated. Deposits under about 20 millionths are microporous. Deposits over about 25 millionths are microcracked. Under some conditions these trivalent chromium deposits might not need particle nickel to obtain the desired number of Active Sites. If micro-discontinuity is not induced, hexavalent chromium will typically macrocrack (visible to unaided eye) in service if plated over 20 millionth in thickness. Most chromium specifications requiring corrosion protection speci- fy between 0.25 to 0.5 microns of chromium (10 to 20 millionths). Hexavalent chromium processes labelled as "Crack-free" deposits will typically macrocrack 185

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