FTM 183-why did Monel dissolve during passivation

Fastener Training Minute 183

This edition of the Fastener Training Minute with Carmen Vertullo was originally published December 15th, 2022 as “Why did my Monel parts dissolve during passivation?” during episode 184 of Fully Threaded Radio.

Well, hi, everyone this is Carmen Vertullo with the Fastener Training Minute coming to you from the Fastener Training Institute and The AIM Testing laboratory here in beautiful El Cajon California, where it is truly a beautiful December day. And for those of you who are listening probably in January, it’s probably a beautiful day today as well. So come and see us sometime.

Today’s topic, as with most topics, came from an email from one of our clients and they had something really bad happen. Their parts dissolved – dissolved. Can you imagine that? what could be worse? Not only did their parts dissolve, but they were expensive parts, and so I’ll tell you the short story, and then when I come back I’ll give you the details and we’ll learn some stuff together, because I learned a few things in this process as well.

So I get this email it says “hey Carmen, we sent some parts out for passivation and they dissolved, and the parts were made out of Monel

Now Monel is a commercial name for a specific nickel copper alloy. Monel is mostly nickel, 65% to 70%, and a lot of copper, 20% to 35% depending on the alloy, with a few other spices thrown in there for good flavor. It’s a very corrosion resistant metal actually, and when we come back I’m going to tell you some stuff about Monel, and I’m going to tell you some stuff about passivation, and hopefully make it so you will never have your parts dissolve.

Well welcome back everybody this is Carmen Vertullo with the Fastener Training Minute, part two. We were talking about an experience that a client had where they sent their Monel parts out for passivation and they dissolved. But let’s first talk about Monel. Monel is a nickel copper alloy, as you heard before, and and it’s very corrosion resistant. It’s also very expensive. And it’s been around for a long time. As a matter of fact before we had stainless steel, which would have been I think stainless steel was invented in the early part of the 19th century, about 1915 or something like that, but up until the 1930’s when stainless steel became more prolific, Monel was to go to alloy for corrosion resistance. As a matter of fact, I recall being on the Queen Mary, it’s it’s an old cruise ship docked up at long beach. All the stuff on that ship that would normally be made out of stainless steel, was made out of Monel. They had things like salt water plumbing on that ship, and everything was Monel, so it was an important metal back then. It still is today, but it was very important metal back in the day.

So Monel is very corrosion resistant. So obviously when the client sends me this this very sad story and I have a lot of questions., my first question is: why are you passivating Monel? Passivation is a process that for stainless steel does two things. It removes any free iron from the surface. It might be called free ferrite. And then it makes the surface active by going into an acid bath, and then the surface becomes immediately active. That is, it has got no chromium oxide on it because it’s been dissolved, and it’s just dying to get a new layer of chromium oxide on it. And as soon as it comes out of the acid and gets exposed to oxygen, boom that layer of chromium oxide is instantaneously there and that’s where the stainless steel gets its corrosion resistance from, and that’s why we passivate as well as because sometimes even though the stainless steel itself will not corrode, the free iron that’s on the surface that can get there a lot of ways, primarily through tooling and handling and other steel that it’s in contacted with I don’t know if the if the if the iron can actually come out of solution somehow on the surface from the stainless steel or not. But basically if we don’t passivate stainless steel we can end up with unsightly surface corrosion,  so that’s why we do it.

Most fastener standards that are made for stainless steel either require or recommend passivation. But Monel is not stainless steel and although it should be cleaned (because it can certainly have the unsightly rust on it because of any free ferrite that might be left behind), it’s not really subject to the passivation process because it has no chromium in it and passivation really is the process of reestablishing a fresh layer of chromium oxide on the surface of the stainless steel.

So what happened is that somehow, and I don’t know the backside of the story, I’m still trying to find out where the requirement for passivation of this Monel come from. I don’t know of any standards that deal with passivation of Monel. There are plenty that deal with pickling and cleaning of it and in fact in my shop. we’ve made parts from Monel we’ve made weldments for Monel. I’ve seen castings and forgings for Monel and they generally need to be pickled afterwards to clean any kind of scale or contamination from them. But in our fastener world when it comes to passivation what we’ll see in most of our stainless steel fastener standards that call out passivation such as ASTM A962 which comes directly out of A193 and A194 for example or F593, these three say that the stainless steel bolting component shall be passivated in accordance with ASTM A380 or A967 at the option of the manufacturer.

We can also do something else called electro polishing which is a super passivation I’m not going to talk about that right now, but it is more than passivation. So this process, if we look at ASTM A380, is very complicated. It has lots of testing things in it. If we look at ASTM A967, it’s a little less complicated, and that would be the option that I would go for. However, interestingly enough. neither ASTM A967 nor ASTM A380 say anything about the alloy that you’re passivating and the problem that we had with the Monel. And here’s the thing you want to remember that we had with the Monel, is that even though it’s very corrosion resistant, it’s actually very highly chemical, chemically resistant. It has an achilles heel. There’s some kryptonite out there that will kill Monel and that kryptonite happens to be the very acid that we use to passivate stainless steel. Stainless steel bolting or any stainless steel part and that acid is nitric acid and any Monel will dissolve very quickly in almost any concentration of nitric acid.

By the way, Monel looks very similar to stainless steel. So if you sent your parts out and say please passivate these and they land at the passivation facility, it might take an experienced person to tell the difference. You might be able to tell the difference by weight having one in one hand and one in the other, because Monel is much heavier than stainless steel. But it looks similar, so when they show up at the passivation facility, and they say oh, we got some stainless steel parts. Let’s passivate these. Sadly enough, when the parts go in the acid and the basket comes out, all that is left are little pieces, because Monel cannot withstand nitric acid, and that’s what those standards require.

Now later on in another Fastener Training Minute, we’ll go into more detail about passivation but just know there’s a lot of detail not only in the so the practice of passivation, but the preparation, the qualification, the testing of the maintenance of the solutions and the process generally. You have got four or five different choices that involve the strength of the nitric acid, the temperature of the nitric acid, and the time that the passivated material is in the nitric acid, and ASTM A967 leaves that up to the passivator or whoever is specifying as to which type of passivation.

And some different stainless steel alloys are susceptible to dissolving as well if you leave them in too long or you don’t use the right solution. So for example, if you send out some 17-4 phase hardened or some 410 stainless steel and you don’t tell them what it is, you could end up with dissolved parts as well or at least partially dissolved parts. Generally our 304 and 316 stainless steels are the least susceptible to that overpassivation problem. All the other ones that are not austinetic stainless steels are susceptible. But there’s also another good side of the story and that is that within ASTM A967 even though it does not say anything in there about which alloys we should use with which types of passivation, it does have another alternative which is citric acid passivation which is becoming much more popular these days. It’s much safer. It won’t hurt Monel, it won’t hurt anything. Some people don’t like it, they think it’s not as good, but it is good. You can actually buy the citric acid passivation solution in a spray bottle.

I don’t know if you’ve noticed lately, but in the last 20 years, a product on the market called stainless steel cable railing has become very popular. And sometimes those cables even though they’re made out of 316 stainless or 304 stainless, they will corrode. The reason is because that wire gets pulled and drawn and it’s all over the place with high high speed steel tooling so that steel transfers off onto the cable, and now the cable is subject to corrosion on the surface. You can fix this very easily with a spray bottle. I think it’s called Citrus Safe or something like that. So citric acid passivation is probably the better way to go.

Now I’ll just tell you one more thing if you need advice about which alloys can be used with which types of passivation solution. You can go to an old federal standard called QQP 35C. Now that standards has been withdrawn and actually it’s been replaced with with ASTM A967. So you can you can use whichever standard you want, but the QPP standard fortunately has a very nice list of all the alloys and which types of passivations that would be appropriate for them. So there is a safe way without having to depend on the passivator or an engineer or getting a consultation if you’re doing much passivation, QPP 35C you should have it in the list. And even even those are recommendations. They’re not requirements.

So if you send your material out to your passivator to be passivated, the most important thing to tell them is what the alloy is, and if they know, they will probably be careful about not leaving it in too long or using the wrong solution. Or just specify that citric acid passivation should be used and you will be safe. QPP 35C lists many stainless steel alloys as a matter of fact I don’t think there’s one that’s not listed in there. However, Monel is not there, so we can rest certain that Monel is not expected to be passivated by any standard at least that I’m aware of. If someone else has some information about that, I would like to know.

Well I hope you learned a little bit about Monel, and a little bit about passivation of stainless steel, and how to avoid some of the pitfalls of it. By the way, this is not the first time I’ve heard this. This is the first time I’ve heard it regarding Monel but I’ve heard it many times regarding other alloys of stainless getting ruined in the passivation process.

Well this has been Carmen Vertullo with your Fastener Training Minute. Thanks for listening.

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