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The Quarterly Newsletter of AlumiPlate, Inc. Plating With Aluminum |
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At AlumiPlate, we've made it a priority to learn as much as we can about aluminum plating, cadmium plating, other coatings, their applications and limitations. We use all of this information to recommend to our customers the best coating solution for their application. In this and upcoming issues of Aluminations, we will offer a series of articles that will try to answer some frequently asked questions. As space constraints don't allow us to present voluminous data that have been compiled on these coatings, we encourage you to contact us with specific questions. Previously, we discussed the reasons that cadmium plating has been targeted for replacement or elimination. This effort has been ongoing for several years and many cadmium alternatives or so-called "cad replacement" coatings have been developed and tested for the myriad applications that cadmium plating has traditionally been used on – critical components (primarily steel) that must not corrode prematurely in extremely corrosive environments. Here, we discuss some of the primary cadmium alternative coatings including zinc alloys and IVD aluminum. The article, to be continued in our summer issue of Aluminations , will also examine organic coatings and AlumiPlate aluminum. Overview The selection of a coating for a particular
application can be a critical design step. Much evaluation, caution and care
is taken by many engineers in this effort. Availability of directly
applicable test data is limited and there are many choices. Targeting the performance of cadmium plating is somewhat subjective – Can we really say that cadmium is the ideal coating? All of the coatings discussed here can, although not necessarily consistently, last for 1,000 hrs in an ASTM B117 salt fog test without red rust. (AlumiPlate aluminum, chromated, has been tested to over 7,000 hrs in B117 with no red rust). Threaded fasteners coated with any of these coatings materials will require a friction modifying lubricant to achieve the desired torque/tension relationships. As George Shaw of TACOM stated in his recent paper published in Products Finishing magazine (Feb '99), "The test data indicate that there is no direct cadmium replacement for all engineering applications." Some of the Alternatives Zinc Alloys: The first coatings to get evaluated as Cadmium
alternatives were the ones that used familiar, water-based chemistry and
existing plating tanks and equipment to obtain a thin corrosion resistant
plating. The various zinc alloys – Zn/Ni, Zn/Fe, Sn/Zn and others have been
available for many years. Strong support from major aerospace interests helped
Zn/Ni become (relatively) widely specified and produced. The zinc alloys are
good corrosion resistant coatings but are extremely sensitive to
"alloy" composition and typically require propriety conversion
coatings. Like all water-based electroplating, zinc alloy platings can cause
embrittlement of high strength steel. It has been reported that galvanic
compatibility of Zn/Ni with aluminum can be negatively impacted by the
apparent "dezincification" of the surface, possibly caused by the sacrificial
corrosion of the zinc component of the plating in a neutral salt spray
environment. IVD Aluminum: Strong support from aerospace interests has also
helped IVD aluminum coating ("Ivadizing") gain some minor success,
despite the fact that the coating is applied in a vacuum chamber rather than
a water-based plating tank. Corrosion resistance is excellent, no
embrittlement is possible and temperature applicability is good. The inherent
porosity of IVD aluminum works against itself in that shot peening is required
to attempt to densify the surface, and the remaining pores can provide sites
for corrosion and stress corrosion cracking in the field. Equipment and
processing costs are quite high compared to the other cadmium alternative
coatings and have limited the acceptance of the coating. |
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