Achieving class 3 chromate conversion coatings

A. Hi TH. There are some interesting aspects to your question. First, if your customer showed you a gold color which is 'perfect' and with no iridescence around the holes, are you absolutely sure the sample was not gold-dyed anodizing rather than chromate conversion coating?

Anodizing is clearly a decorative finish whereas most people consider chromate conversion coating a functional finish rather than a decorative one. So 'perfect' yellow anodizing probably isn't nearly as difficult as 'perfect' yellow chromate conversion coating.

Yes, a thicker conversion coating will probably be less iridescent, and a more consistent color is probably obtainable with lots of agitation than with poorer agitation. And I suppose it is also possible that Henkel has a particular Alodine process which they feel gives a more consistent, less iridescent color.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi TH!
I love this question, because it comes up in my job so often:)

The specs for yellow chromate come right out and say that color variation is normal and acceptable... it's a FUNCTIONAL spec, rather than aesthetic!

But because we like happy customers... here are some things I've encountered that mess up the visual appeal:

Remember always that the chemfilm isn't fully "set" when it leaves the bath, the way you'd expect say.. a black anodize to be. It is fragile and easily discolored for up to 48 hours.

Holes and contours: Bleed-out from liquid in holes or tight contours will show as discoloration.

Rack drips: taking parts off the line, the first step is to blow off the flight bars and racks! Drips will show up, and if you have shared flight bars with a current-added process like anodize, that copper is BAD. It will not only discolor, but also compromise the utility of the finish.

Surface prep: one of the most combative SCARs I've ever dealt with was from a customer who milled parts, then tumbled them, then sent them to us demanding "uniform color". Spoiler, the color was not uniform, because the tumbling did not impact internal contours equally to flat surfaces. We even hacked up one of their rejected parts and had it salt-fogged at 168h ... 0 pits. Again: FUNCTIONAL specification. The surface prep also applies even to slight variations in finish quality (RA? Roughness? Microterrain? I'm not a Milling Inspector haha) on various parts of um ... the part. But seriously, if there's different tooling and different finish on different bits, of course you will see the color/sheen/iridescence varies as well.

If you can pass salt fog, you met the spec... if the PO requires visual appeal, that's a high quote or a no-quote.

Your best bet for a uniform color is an extremely well prepared surface without holes or contours, and an unracking team who know how to approach conversion coating. But it is still a really big ask to demand color uniformity on a spec that very clearly spells out that it is NOT a requirement.

One of the stupidest chromate fails I've seen involved streaking from the parts having been cut on a waterjet with mild steel supports ... they left a visible tattoo pattern on the finished parts ... 1.25" apart :)
I drove over to the source with a camera and a ruler and made a bit of a stink about the garbage in / garbage out thing.

There are a million ways to fail at a "perfect" chromate cc, and it will be a team effort between you and your customer to have them send parts that are even capable of taking a good coating, and your team to provide and protect it until it has cured.

Since the customer does not need RoHS obviously ... how's about a nice Type II anodize with sodium dichromate seal? ;)

A. Hi Richard,

The one bit of help I can give is how to test for the coating weight.

Alocrom your panel rinse and blow or air dry (do not oven dry).

Then find the mass on a lab balance to 0.0001g.

Strip in 50% concentrated nitric 50% water (it will strip quick if within about 3 hours of coating) then rinse.

Swab with damp cotton wool if required to remove traces of coating.

Rinse and dry.

Then reweigh. From the area of the panel you can work out the coat weight.

Also stripping in nitric does not have a go at the base metal unlike other strips.

You will find the relationship between coating weight and time is non-linear, so you will have to test different dip times to find the best time to get the coating required.

Also you will have to put in place some testing as the time will drift a bit due to bath conc., pH, temperature etc.

As to the bit about the customer supplies the requirements you are right two customers could supply two different requirements, so go back and make sure they know what they want before you start supplying products to them. (If I was a betting man I would bet that the customer only wants what they had before of someone else and don't know really what they want)

A. There are only two requirements in MIL-C-5541E that differ between Class 1a and 3.

1) The solution used must be on the Qualified Products List (QPL) for that Class. Many products, including Iridite 14 and Alodine 1200, are listed for both classes.

2) Class 3 must pass a Electrical Contact Resistance Test per paragraph 3.8, but only when specified. I have never seen this specified.

So in most cases the specification makes no required distinctions between Class 1a and 3 coatings.

The only discussion about immersion times, coating thickness, and/or coating weights occurs in the non-mandatory Notes section of the specification. It suggests that Class 3 coatings are thinner due to lower immersion times but gives no guidance.

A. Richard,

What Martin said is absolutely correct. I would like to add, though, that while two classes are different, they don't have to be processed that way. Our customers only require an electrical resistance test (<5,000 micro-ohms before salt spray and <10,000 micro-ohms after) to certify to class 3, since our class 1A w/ a 3 minute immersion time satisfies this test, we can run our class 1A and 3 on the same rack. Our customers also accept one salt spray test for both classes as they are the same thickness/coating weight.

I don't believe coating weight/thickness is the difference between the classes, I believe that the electrical properties are the dividing factor as class 3 needs to be acceptable for EB (Electrical Bond) areas.

Hope this gives some more info.

A. Without an electrical resistance requirement, only four conditions must be met for a Class 3 coating.

1)The product must be on the QPL for Class 3.
2)The monthly corrosion and wet tape testing must be performed.
3)The bath solution must be analysed weekly.
4)The coating must be uniform in appearance and iridescent yellow to brown.

A. You are correct. If electrical resistance testing is not specified, the exact same coating could meet the requirements of both Class 1a and 3. Testing per MIL-C-81706 is not the responsibility of the applicator but rather the chemical manufacturer. The applicator's responsibility to use only QPL products listed for the appropriate class and certified to MIL-DTL-81706 [on DLA] .

A. Hello Bruce,

Per the Mil Spec, Class determines coating thickness NOT color. Class 1A coatings are thicker and as a result less conductive, gold color less conductive than clear. Chromates are thin and can easily be penetrated for electrical contact. Class 3 coatings are usually for paint prep or least electrical resistance. Class 1a for the best corrosion resistance.

A. The difference between a class 1A coating and a class 3 coating is, a class 3 coating has customer specified electrical conductivity values to suit a given application ref para 3.8 of the specification. Essentially a class 3 coating is customer specific i.e., one customer's class 3 coating may be another's class 1A. It is worth noting however that the conversion materials are qualified to a specific set of values, ref section 6 of the MIL specification. Please also note, both class 3 and class 1A coatings (regardless of colour) have to pass the same corrosion tests i.e., 168 hours salt spray.

So, to answer your question, tone or colour does not define the class of coating. However, as a rule a thicker coating has a darker colour, more corrosion protection and higher resistance values than a lighter, thinner coating. That said, colours at both ends of the spectrum must pass the 168 hours salt spray.

A. It's nearly impossible (alright down right difficult then ) to get diecast aluminum such as 380 to pass the 168 hour salt spray REQUIREMENT of MIL-C-5541 even with a good strong etch so my guess, no my experience tells me so, that there is NO CHANCE that the resulting chem film is up to MIL-C-5541 standards without the etch. In fact, to be honest, if you're immersing the entire anodized part into the chem film bath, I'd be fairly certain that the chromating bath would eat into the anodize seal layer and disrupt it to the point where the anodize will no longer pass 336 hour salt spray. My opinion, diecast aren't capable of meeting any finishing standard that includes a corrosion resistance requirement. Good luck.

A. MIL-C-5541E is a process specification. Per Paragraph 4.3.1, the required inspection is a process control test to prove that the chemical conversion coatings "... are acceptable as evidence of the properties being obtained with the equipment and procedures employed." That is, it is only necessary to prove monthly that the process is in control not that each part or alloy coated meets these requirements. Therefore, if parts including castings, are coated using the validated chemicals and procedures, there is no specification requirement that they individually pass or be able to pass a 168 hour corrosion resistance test. For Class 3, this requirement is only for "specimen panels" of 6061-T6 material. Lot acceptance only requires a visual inspection for appearance and workmanship. The specification for testing and listing QPL materials, MIL-C-81706, only tests Class 3 chemistries on 6061-T6.

A. You may want to check into Metalast TCP-HF as a replacement for your Chem-Film coating on the cast part. Developed by the department of the Navy for coating F/A-18 Hornets. Research has showed that Metalast TCP-HF will pass salt spray tests on all cast aluminum. It can even be used to seal the anodized pores and achieve between 500-1000 hours of salt spray for the anodized layer.

A. The "New" tri-chrome for aluminum that replaces the hex chrome chem films is supplied by 3 companies in the USA. All must comply with the original formulation and if followed will produce the same results.

That being said surface pretreatment is most important even with a top of the line product therefore be careful when quoting laboratory results under perfect conditions. What really is important is performance results under most "reasonable" shop conditions. I suggest having a good process/control with quality materials and available, competent, local technical help is best.

Drew Nosti, CEF
Anodize USA

Ladson, South Carolina

Indeed, Drew, even when inventors/developers work for the Navy, pride of ownership can lead people to exaggerate advantages and downplay disadvantages :-)
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Class 3 is NOT the equivalent of class 1a, regardless of what some MFGs say or passing a minimum salt spray.
You need to fill the customers PO / spec or get him to change it. Darker chem film is a lot easier to spot a problem than pale will ever be.

A. Hi, Antonio. I have seen shops get a clear chromate by dipping for just 15 seconds into a yellow chromate, and then leaching the color out by soaking in warm water -- but some people question whether this really affords measurable protection. It is much better to install 2 tanks, one for Class 1A and one for Class 3. Please note that Alodine 1200S is a proprietary product, and Henkel will supply you with a technical data sheet for the recommended times, pH, and temperature.

Regards,

A. Per MIL-C-5541, a clear coating is NOT allowed for either Class 1a or 3 unless specified by the purchaser. When using products that are listed on the QPL for both classes, such as Alodine 1200, there is NO requirement to process the Classes differently. There could be an additional electrical resistance testing for Class 3 coatings, but only when required by the purchaser.