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Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Randy Taylor, Advanced Tooling Corporation.
July, 2008

Industrial Chrome on Hydraulic Cylinder Rods

Q. Good Morning, I am Larry from Western Hydrostatics located in Riverside, CA. We have a hydraulic cylinder manufacturing and repair shop and I am interested in learning more about industrial chrome on cylinder rods. I am currently wanting to find out how hard chrome is and/or should be on rods and if it varies by chrome shop and manufacture, if its a new cylinder. Also, is there a certain mixture and/or process that is an industry standard that all industrial chrome shops should be using? I guess I am also asking if its possible that some shops can cut corners on mixtures(if applies) to save money and chrome does not come out as hard as it should be? Thanks so much ...

A. Larry, I apologize for the delay in answering your email. I've been out of town and am just now getting back into the saddle.

Chromium coatings fall two basic categories, functional chromium and decorative. Functional chrome imparts wear resistance, can vary widely in thickness. It is primarily used to restore worn or undersize dimensions. Decorative chromium imparts a certain appearance requirement and provides corrosion resistance. Functional chromium alone is not necessarily the best choice for corrosion resistance.

In functional hard chrome, there are standard baths that operate in a general range and the bath lasts for years with very little maintenance. And there are specially formulated baths which can impart a variety of superior wear and corrosion characteristics. Standard baths are less expensive to operate and specially formulated baths are more expensive and require more maintenance.

Every plating shop is different, different owners, different customer base, different circumstances, but as a general rule, properly applied chromium coatings are "metallurgic ally" consistent throughout the US and Canada. That said...

It's not cast in stone how a Chrome plating shop approaches a job, but shops that are able to certify their chrome plating process, generally provide the highest quality chromium coatings. It's not likely that a plater would cut corners regarding the bath. A chemically weak or dirty bath would still produce a decent chrome deposit but would require longer plating cycle time, more energy consumption, costing more and reducing throughput.

Chrome deposit quality can certainly be "substandard" but that's not necessarily caused by what's being done, but rather by "what's not being done" and maybe should be.

If no other requirements or special parameters are called out on the contract or order, a general process of applying chrome on steel would likely only include a manual degrease, a visual check for dimensions and metal condition, mask, pre-etch and chrome plate. There's no guarantee a plating shop will concern themselves with coating performance issues unless they are discussed in advance. There are several process steps that could be added up front which can greatly improve the performance of the coating such as shot peening, grit blasting, polishing, pitting and micro finish inspections, even stress relieving. And there are wet process procedures which can enhance the chrome deposit characteristics like temperature, agitation and anode conformance and spacing just to name a few.

In our industry the general consensus by engineers is that "chromium" is universally corrosion resistant. The fact is chromium applied directly on to steel with no special preparation and/or supplemental process parameters, may not be as corrosion resistant as one might think.

There are many factors affecting the ability of chromium by itself to resist corrosion. The more common are bath chemistry, part preparation, plating current density, thickness and uniformity. It's not well known, but in fact it's quite normal that the majority of functional chromium coatings tend to crack after plating. A dense population of small shallow micro-cracks is the most desirable. On the flip side, too many larger macro-cracks (cracks that go all the way thru the coating) can invite corrosion. Cracks in chromium can be controlled in part by several factors, tank temperature, coating thickness and current density. Controlling and even eliminating chrome deposit cracking is a key element to the successful corrosion resistance of chromium and the topic is rarely discussed and often overlooked by engineering. In extreme environmental conditions where corrosion can be the most severe, an underlay of copper and nickel coating may be needed to supplement the corrosion resistance of chromium.

As a customer going out to the industry and requesting chromium, it's important to establish a dialog with the plater and discuss chromium coating requirements as well as their individual coating performance track record. Most reputable plating shops will be able to provide answers and information about coating performance and some will provide the added reassurance of a process certification.

Best of luck.

Randy Taylor

 



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