August 5, 2020
You probably already know what a certificate of conformance is: it’s a written guarantee from your processer declaring that your parts have been processed in compliance with the relevant specification. But did you know that not all certs are created equal? In this article I will cover the questions you should ask your anodizing supplier to ensure that they are fully compliant with the requirements. You might be surprised to find many are not.
Like any manufacturing process, anodizing requires testing and measurement to ensure compliance to the specification. In this instance, I will talk about MIL-A-8625, the most common anodizing specification. There are two different types of testing required by the specification: Lot Testing, and Periodic Testing.
A lot, or batch, of parts is comprised of a group of parts that are processed simultaneously. All the parts in a lot must have the same part number, and they must all be processed during the same shift or else they shall be considered separate lots. So, here is your first question: how many certs did you receive? If you sent thousands of parts and it took several days for your supplier to finish them, you should expect more than just one cert. In other words, if they broke the job into three lots, you should get three certs.
Each of the lots should have been subjected to a number of inspections: Visual, dimensional, and paint adhesion. Your supplier may use a sampling plan for these inspections, but that also requires documentation which you are entitled to request and review.
Anotek doesn’t use sampling plans for visual inspection: we inspect 100% of all parts. It’s a straightforward inspection: it simply requires that the parts look good! (I’ll talk about the actual inspection criteria for lot testing in another article.)
Dimensional inspection, which ensures the anodizing process has not altered the shape of the part is a standard inspection but many shops do not perform it! Permission to waive dimensional inspection must be called out on the purchase order. It is rare for a customer to require this inspection as it would only be useful on high tolerance surfaces and holes, but those are normally protected by masking or plugging, or designed with an allowance for the anodic coating growth that occurs during the process. Furthermore, if you want the inspection done, you need to specify the methodology and the acceptance criteria.
The final inspection is paint adhesion, and again, is only required if the parts are painted after anodize. This is a critical test that is often omitted or done improperly, resulting in the paint flaking off your parts after they have gone into service. That does nothing good for your reputation!
Periodic Testing – also known as Process Control Testing - is a set of tests that ensure your supplier’s process is in compliance with the requirements of the specification. Unlike some anodizing specs, such as Boeing’s BAC5632 boric-sulfuric acid anodizing process, MIL-A-8625 contains virtually no instructions on how to set up your process. The only thing it really requires is that you use sulfuric acid (or chromic acid for Type I and IB). Everything else is left up to the supplier to design themselves. This is the main reason you might get a different looking finish from one supplier compared to another. It’s also the reason why Periodic Testing is so important: the process can be variable, but the results cannot. So, if your supplier is not conducting all these tests, there is no way to ensure your parts conform, and therefore the cert is not worth the paper it’s written on. It’s basically fraud.
MIL-A-8625 is a wet, immersion process where your parts are dipped in a series of various tanks. The spec does not demand the use of specific materials but it does demand that your supplier create a Process Control Document (PCD) to define what solutions he/she is using, the parameters of each of those materials (ie. concentration, temperature, immersion times, voltage), and that they have a defined plan for how your parts will be processed. (You have the right to review that process!) Your supplier must test all of their solutions at a minimum frequency of two weeks to confirm that they are within the parameters defined in their PCD. (You have a right to review their test results.) If any of their results are outside the ranges defined in their PCD, their process is non-compliant and, unfortunately, so are your parts. You might also ask whether your supplier ever confirms their testing with an outside lab. All of Anotek’s testing is conducted by a third-party, Nadcap accredited laboratory so there is never any question about the credibility of our results.
We often get asked about the thickness of the anodize coating on parts and customers look at us funny when we say we don’t know. The reason for that is there is no requirement to measure the thickness of the coating (see dimensional inspection, above); we measure the weight of the coating instead. This is done by running some sample pieces (called coupons) of a prescribed size, then chemically removing, or stripping, the coating from the coupon. The amount of coating, by weight, that we can remove, determines whether it is compliant with the spec. This test must be done monthly at a minimum.
This test is the bane of all anodizers and if your supplier is shirking, this is probably the place. This test requires the creation of five coupons that are then placed in a test chamber filled with a salty mist. They must remain there for 336 hours (two full weeks!) after which they are evaluated for corrosion and pitting. It’s a long, expensive and difficult test to pass and must be done, at a minimum, every 35 days. Any wonder why some anodizing shops don’t bother with this test? There are plenty of suppliers that say they are “generally compliant” with the spec but corrosion resistance is one of the main goals of anodizing and it makes me wonder why anyone would do business with a shop that ignores this testing requirement. (And yes, you have the right to inspect their records!)
This test is rarely required and must be requested by the customer. It is only performed on dyed (ie Class 2) parts and involves exposing processed parts to UV radiation for a period of time to see whether the dyes fade at an unacceptable rate (some fading is inevitable).
This is only applicable to Type III, but can also be replaced with the coating weight test. Due to the variable geometry of most parts, coating thickness varies according to the location on the part, and therefore you can get significantly different thickness readings on different surfaces. Once again, this is the reason for testing coating weight.
Also applicable only to Type III parts, this test essentially subjects test pieces to a controlled abrasive process to see how long the coating withstands the contact. If the coating wears away too quickly, the result is a test failure.
The consequence of failing a Periodic Test is severe. All parts processed since the last successful test are considered non-conforming and must be recalled. Furthermore, the supplier must stop processing until a successful set of process control tests can be completed, which you now know takes a minimum of two weeks. Ouch!
As you can see, a certificate of conformance represents a lot more than just a supplier’s assurance that everything is fine! Compliance with the specification is difficult and a cert is your supplier’s guarantee that ALL the requirements of the specification are met, not just the ones your supplier chooses to meet. This is one of the advantages of choosing a Nadcap accredited supplier like Anotek. When a third-party comes and audits your process, you have to have it right – all of it.
For a free consultation on your anodizing, chem conversion, NDT or primer requirements,
please call Anotek at +1 (604) 459-2868.
19483 Fraser Way, Pitt Meadows, BC, V3Y 2V4, Canada