Suitable replacement for hard anodize on aluminum parts

[dans95]

We currently use hard anodize on aluminum parts (Mil-A-8625F, Type III, Class 2). The anodize gives acceptable corrosion resistance and cosmetics. However the process ends up being expensive becuase of the extensive masking required. Due to the build up (.001")small threaded holes must be plugged and because the anodize is non-conductive various areas must be masked. Mostly due to the labor involved the plating accounts for 20-30% of the total cost of the aluminum parts.

What platings can be used on aluminum which:
1) Provide equivalent (or better) corrosion resistance than hard anodize (product is used in medical/research labs and subjected to decontamination with bleach)
2) Is conductive.
3) Is thin enough to not require plugging small (M2-M4) threaded holes.
4) Provides decent cosmetics.
5) Cost competitive with anodize (without the masking labor).

 

[unclesyd]

The masking problem limits you to a conversion coating or a CVD. Your application is a tricky one as medical cleaners can get quite aggressive. The problem is that you design for a particular class of cleaners and they immediately change the process when they get your part.

Here is one coating that may have possibilities that I've seen used with some success.

Hit the radio buttons for more information. I would also give them a call.

http://www.sanchem.com/safegard_cc.html

Here is another possibility. We use this coating on several projects both as is and anodized.

http://www.alumiplate.com/index.html

 

[dans95]

Thanks for the input.

If the coating is thin enough and conductive there won't be a need for masking.

 

[edrush]

Why are you hard anodizing? I assume because you need wear resistance. Decorative anodizing (Type II) has as good or better better corrosion resistance and is typically thinner, but does not nave the wear resistance.

This is a challenging problem and I share your pain.

You can use custom taps so your threads will be the correct size after plating, but that may be where you need your conductivity.

You could move to a 17-4PH which may be able to compete with your aluminum on a strength-to-weight basis, but in many circumstances you will need some part volume where you don't need strength, so it's difficult to match the weight of aluminum. And aluminum is a easier to machine. I assume titanium is out of the question due to cost, but on the other hand medical applications can usually tolerate higher costs than consumer products.

Chlorides will eventually dissolve anodize.

Any chance of using a sacrifical anode?

I tried nickel because it has great wear properties, but discovered (painfully) that unless the coating is perfect, that emf series problem will get you. And there is no such thing as perfect.

 

[edrush]

I meant electoless nickel plating above.

 

[dans95]

We are using hard anodize for corrosion resistance. The limited research we've done and discussing the issues with the plater led us to choose hard anodize. According to the plater the increased thickness of type III is what gives it better corrosion resistance compared to type II.

Weight and machining costs limits the use of steel in this application.

Emf series problems? I'm not familiar with that term, please elaborate.

 

[edrush]

Dan,

I mean galvanic corrosion. If you plate aluminum with most any metal the aluminum corrodes before the other metal. If the plating is the metal exposed to the environment the interface tends to corrode (if any flaws or cracks are present)and the plating flakes off.

 

[edrush]

I wish it was possible to edit posts on this site.

Is your Type III anodize sealed?

How many conductive areas do you need? Usually just plugging one hole is enough - not a big deal. This must either be complex masking or a very high volume part. If it's medical, I have trouble seeing the potential for volume.

Is you current process working well (besides the cost issue)? If it works, is it worth changing the process to save $2-3/part?

 

[dans95]

In response to your questions:

1) Yes the type III is sealed.

2) The product uses a capacitive contact sensing system which is particularly sensitive to grounding issues. Large pieces of metal which are "floating" electrically cause problems. The EE's didn't want to rely on threaded holes for grounding so various contact areas are also masked. With the numerous threaded holes, bearing bores, and contact areas the masking is somewhat complex.

3) The anodize provides acceptable results, not great. The most costly part to plate on this project has 50+ tapped holes (we originally tried not plugging these but found the inconsistency in the small sizes, M2-M3, to be problematic)and few masked contact areas. The cost breakdown is ~$40 for material, ~$75 fabricating, ~$85 for plating! (most of the plating cost is masking labor)

Anyone have good results from PVD coatings or some type of nickel coating?