Triplex nickel

A. Hi Jim. Duplex nickel has been well understood for decades. It relies on the fact that the outer layer (the bright nickel), is higher in sulfur than the inner layer of semibright nickel, so it corrodes preferentially. The bright nickel offers cathodic protection to the semibright nickel, spreading the corrosion forces laterally instead of allowing them to penetrate the semibright layer, and on to the destruction of the underlying steel. The Chrysler-designed STEP test can help assure that you are operating at optimal electromotive potentials.

Once you go beyond duplex nickel you are definitely into highly proprietary automotive specs, which can include a high sulfur interlayer which (hopefully) corrodes in preference to either the bright or the semibright layers; and there are other corrosion-proofing measures like microcracked or microporous chrome, "pixie dust" layers, etc.

If you are plating OEM automotive exterior components or contemplating doing so, the automotive specs and the proprietary knowledge of the major nickel process suppliers can give you examples. But I personally find it hard to believe that proper thicknesses of properly operated duplex nickel plating could fail to deliver the required corrosion resistance for any application other than exterior automotive.

Regards,

A. Ted: You say - "But I personally find it hard to believe that proper thicknesses of properly operated duplex nickel plating could fail to deliver the required corrosion resistance for any application other than exterior automotive"

I think I am correct in saying that triplex nickel is no longer seen as having any great advantage over duplex. It was, twenty or thirty years ago, somewhat fad-ish, but I'm reasonably certain that now duplex is far more widely used, including exterior automotive applications.

Thanks Jeffrey. Duplex nickel is the only automotive exterior finish I'm actually familiar with. I saw auto and truck bumpers and trim duplex nickel plated in more than a dozen shops in my career, but don't remember if I actually ever saw triplex being done. Although I've seen it in automotive specs, you may be right that those specs are now outdated; I don't know.

Regards,

Thanks for the help. We're not aiming for exterior automotive trim, but we have applications for the plating that are as demanding in terms of corrosion resistance.

A. Jim - I can contribute this much: The first major use of duplex nickel I am aware of was 1965 Ford on zinc die castings. The spec at that time was .0006" semi-bright overplated with .0003" bright nickel followed by conventional chrome.

That was a large advance in corrosion resistance. These days I think nickel thicknesses are a little more, perhaps .001"/.0003", and the chrome is usually microcracked/microporous.

Fifteen to twenty years of continuous exposure (bumpers, etc) without any rusting is common. Just look at all the old trucks on the road with still bright any shiny bumpers.

A. I used to plate OEM components for quite a long time and this what was called out on all Big 3 specs -- the above explanations really explain well how the complete system works.

Triple or four layer systems are used, semi, high sulfur (optional), bright and microporous and then hex chrome; check ASTM B456 for the different layer thicknesses.

Using the optional high sulfur layer you could decrease the total nickel thickness as much as 25% but you had to keep certain SB-B ratio; this bath is quite easy to control. The microporous nickel is quite tricky to operate but is favored by the US manufacturers while the microcracked is preferred by Europeans.

There is another way to induce micropores into the chrome layer and it can be done by blowing aluminum oxide at a low pressure onto the finished part (similar to sand blasting); you have to be careful as over-exposing can create a dull effect on the part. I actually saw this process in a large plating shop and also did it myself for quite a long time.

A. Tri - nickel is out.

Microporous nickel and microcracked Ni is in.

These two nickel layer are used (together or separately) for the automotive industry to overcome what is called - Russian Mud.

Russian Mud is a new test for corrosion, much more demanding than NSST or CASS.