Stainless Steel corrosion issue

Not my issue directly, more a case of relaying the problem, but here goes.

My girlfriend works for a company that makes bioanalytical units, and is part of their validation team in particular. They’ve started running into an issue with a stainless steel solenoid valve where it is, for lack of a better description, rusting. It’s producing a slurry or sludge that is definitely iron oxide color, and there’s visible pitting on the inside of the unit. Some tests have conclusively tied the issue to exposure to hydrochloric acid that’s used as part of the cycle in the analysis unit. All attempts to reach out to the vendor who makes the valves with questions about the metallurgy of the valves have failed, and the team is scratching their heads over it.

Is there any way to determine what is going on at a chemistry level to cause it, or to find ways to protect the valve from this corrosion?

What is the grade of the stainless steel?
304 is easier to machine, while 316 is designed to survive in the presence of chlorine and seawater.

Is the colour due to corrosion and pitting of the surface, or deposition onto the surface?

Stainless steel is only stainless in the presence of oxygen. Without oxygen, in the presence of sulfur, it crumbles to a black grit. Never use it for bolts to hold the keel on, in a bilge.

You must consider the full chemical cycle, to analyse how the surface may be activated by one chemistry, to be susceptible to another. Does HCl activate the surface by destroying the Cr or Ni oxides, that would normally passivate the surface in the presence of oxygen.

When iron is present in water, due to corrosion of other iron components, that iron can come out of solution on the surface of stainless steel, to form a red or yellow rust-coloured surface.
https://corrosion-doctors.org/MatSelect/rouging.htm
https://www.enerquip.com/rouging-what-it-is-and-how-to-avoid-it/

Baluncore said:

What is the grade of the stainless steel?
304 is easier to machine, while 316 is designed to survive in the presence of chlorine and seawater.

Unknown. That’s why she was reaching out to the manufacturer for more info, but has been unsuccessful in getting a response.

Baluncore said:

Is the colour due to corrosion and pitting of the surface, or deposition onto the surface?

The sludge deposits on the plunger side of the valve and the spring and is easily removed with wiping. The body of the valve undergoes pitting.

Baluncore said:

Stainless steel is only stainless in the presence of oxygen. Without oxygen, in the presence of sulfur, it crumbles to a black grit. Never use it for bolts to hold the keel on, in a bilge.

It is an anoxic environment, as it’s constantly flooded internally with water, acid, etc. The sulfur is an interesting point though, and I’ll ask if there’s any sulfur-bearing solutions involved.

Baluncore said:

You must consider the full chemical cycle, to analyse how the surface may be activated by one chemistry, to be susceptible to another. Does HCl activate the surface by destroying the Cr or Ni oxides, that would normally passivate the surface in the presence of oxygen.https://www.enerquip.com/rouging-what-it-is-and-how-to-avoid-it/

That’s the weird bit. She tested a brand new valve in HCl only and it manifested in about three days. No other chemicals were run through it and it still happened. Hence why I said it’s conclusively tied to the acid. And it’s only the valve body, not the plunger or spring.

I did ask about dissimilar metal corrosion early on, but I seem to recall the testing included just dunking the parts in a beaker of 0.1 molar hydrochloric acid and leaving it for a couple days and then getting the same result. I’ll double check with her on that tonight, though.

Flyboy said:

Hence why I said it’s conclusively tied to the acid. And it’s only the valve body, not the plunger or spring.

The spring and plunger may be SS grade 316, made from wire, or ground from stock bar. 316 work hardens instantly when being cut, so is best ground to profile, with a carbide grit wheel.

SS grade 304 is not resistant to chlorine, but can be cut easily, (I think because they add a touch of sulfur). Since the body needed to be machined, it was easier to use 304. I expect the manufacturer will not want to admit to that mistake. Maybe the manufacturer needs to use a different composition carbide or ceramic cutting tool, so they can machine the bodies from 316.

Baluncore said:

The spring and plunger may be SS grade 316, made from wire, or ground from stock bar. 316 work hardens instantly when being cut, so is best ground to profile, with a carbide grit wheel.

SS grade 304 is not resistant to chlorine, but can be cut easily, (I think because they add a touch of sulfur). Since the body needed to be machined, it was easier to use 304. I expect the manufacturer will not want to admit to that mistake. Maybe the manufacturer needs to use a different composition carbide or ceramic cutting tool, so they can machine the bodies from 316.

Is there any way to test between the two alloys without destructive testing, or without sophisticated metallurgical equipment?

Flyboy said:

Is there any way to test between the two alloys without destructive testing, or without sophisticated metallurgical equipment?

Not really. XRF may identify the Cr and Ni content, but the presence of minor components makes the real difference to the corrosion properties.
316 <= 18% Cr
304 >= 18% Cr

316 is usually stamped on items made from 316, and they cost twice the price of a similar item made from 304.

Baluncore said:

but the presence of minor components makes the real difference to the corrosion properties.

316 has 2-3% Mo, while 304 does not specify Mo, but could have some, and probably < 1%. Mo acts synergistically with Cr to improve pitting resistance in the presence of chlorides. One might look at the PRN for the stainless steel grade (PREN = Pitting Resistance Equivalent Number), and there are plenty of higher PRN stainless steels if one uses stainless steel in seawater or HCl solutions. In some environments, 317 and 317LMN are even better than 316, and then there are specialty stainless steels, e.g., AL-6X and AL-6XN, XM-19 (Nitronic(R) 50, 22-13-5), and others, e.g., 904L, 254SMO, 4565S. Most have nitrogen (N) added, and the latter have higher levels of Mo than 316.

PREN - https://www.unifiedalloys.com/blog/pitting-resistance-pren (provided for information only. No endorsement expressed or implied). The information may be used for comparative purposes, but the user is responsible to ensure choice of alloy is appropriate for the intended environment.
See also - https://en.wikipedia.org/wiki/Pitting_resistance_equivalent_number

Sulfur (and phosphorus) are impurities in stainless steel, and should lower than allowances in commercial standards. S and Se may be added to austenitic stainless steel to improve cutting, but that comes with potential corrosion/pitting issues depending on the chemical and thermal environment.

What is the typical process temperature during exposure to hydrochloric acid? What concentration? What is the pH range?

One might use a portable XRF device (e.g., https://elvatech.com/applications/alloy/) to look at the composition (look for Mo). Otherwise, there should be a certificate for the part/material. (One should always make sure to have certificates (of compliance) in contracts or purchase order.).

https://www.olympus-ims.com/en/learn/xrf/applications/metal-alloy-pmi/
https://www.bruker.com/en/products-and-solutions/elemental-analyzers/handheld-xrf-spectrometers.html

One can search the internet for "Portable XRF Analyzer", or replace "Analyzer" with "analysis for alloy composition"

Baluncore said:

Since the body needed to be machined, it was easier to use 304. I expect the manufacturer will not want to admit to that mistake.

Do we know if the purchaser provided a spec for this valve? Does the manufacturer even know their valve is being used in HCl service?

gmax137 said:

Do we know if the purchaser provided a spec for this valve? Does the manufacturer even know their valve is being used in HCl service?

No, and unclear. My girlfriend told me that she has finally gotten in touch with an actual engineer at the vendor, but so far the responses have been vague enough to be infuriating. Saying it’s made with “300-series stainless steel” is like saying that my truck has a V-6 engine in it. Also something about 450FR, I think? She’s still hounding them for more details.

I did learn more about the lead up to the issue in the first place. The unit in question was never designed for use with HCl, but they’re experimenting with a new process for it that does require HCl. They considered other options for the acid, but they were either incompatible with the chemistry of the process, hazardous to handle, or were known to attack parts of the equipment. The reaction of the valve body with the HCl was an unexpected and unwelcome complication.

There is another system they’re starting to field that has limited exposure to HCl, to the tune of about once a year, and only for a few minutes, and that’s optional, but they’re watching this particular valve issue closely to see how it shakes out, as it might need to be addressed on the newer system.

Astronuc said:

What is the typical process temperature during exposure to hydrochloric acid? What concentration? What is the pH range?

One might use a portable XRF device (e.g., https://elvatech.com/applications/alloy/) to look at the composition (look for Mo). Otherwise, there should be a certificate for the part/material. (One should always make sure to have certificates (of compliance) in contracts or purchase order.).https://www.bruker.com/en/products-and-solutions/elemental-analyzers/handheld-xrf-spectrometers.html

It’s at room temperature or a little cooler, like in a climate controlled laboratory setting. Concentration is ~0.1 molar, iirc.

Not sure if the portable XRF is in the cards yet. They might have to look into it if they can’t get specs from the vendor, but at that point, they might as well just source a new valve and issue that to the field service reps for retrofit on units expected to work with the acid.

A portable XRF is an expensive tool.
Take a valve body to a mineral analyst, or to a big metal scrapyard where they have an XRF for sorting steel. These days, XRF should be able to identify the particular alloy, from its stored database.

Look for a similar rated valve made from 316.

Maybe get the valve body cloned from 316, specifically for your application.
A shop equipped with thread grinding and wire EDM might help.

Flyboy said:

Not sure if the portable XRF is in the cards yet.

Alternatively, as Baluncore suggested, find a lab with an XRF for a quick analysis.

Flyboy said:

if they can’t get specs from the vendor,

Yes, see if that can get the specs, and as one suggested, perhaps time to source a new value with different material - suitable for HCl exposure.

Finally, some progress!

She got a response with actually useful data…

“The valve body is 303 stainless steel, the plunger is 430FR stainless steel with a 304 ss spring.”

Soooo… yeah, that sounds like a case of needing a new valve if they want to run HCl?

A useful reference, which should be taken 'for information only'. One is responsible for one's due diligence in selecting an appropriate alloy for a given environment. In general, it is preferable not to have dissimilar metals in contact or in close proximity in the same electrolyte.

https://nickelinstitute.org/media/1702/highperformancestainlesssteels_11021_.pdf

Well, the adventure into metallurgy has come to a close. New valves have been ordered, and I quote:

“Based on the required pressure, they recommended a metal valve with nitric passivation.”

They’re not expecting to receive them for a month or so, as they’re manufactured on demand, but it sounds like they’re not going to cost much more than the original stainless ones.

Thanks for the insight, gang.

With regard to the alloy 303 used for the body of the original valve ...
Penn Stainless Products advise ... https://www.pennstainless.com/resources/literature/
Alloy 303 has good resistance to mildly corrosive atmospheres, but significantly less than alloy 304. When additional sulfur is added to improve machinability, it can often slightly reduce corrosion resistance. Like other common austenitic stainless steels, it is subject to corrosion cracking in chloride environments above 60ºC.
Alloy 303 should not be exposed to moist, marine environments as it is subject to rapid-pitting corrosion and a rust film will tend to form.

Baluncore said:

Alloy 303 has good resistance to mildly corrosive atmospheres, but significantly less than alloy 304. When additional sulfur is added to improve machinability, it can often slightly reduce corrosion resistance. Like other common austenitic stainless steels, it is subject to corrosion cracking in chloride environments above 60ºC.
Alloy 303 should not be exposed to moist, marine environments as it is subject to rapid-pitting corrosion and a rust film will tend to form.

304 would be subject to mild corrosion and pitting in a marine environment, more so than 316, for example. Similarly, for chloride environment.

In ambient - normal outside or room temperature - a little sulfur is not a concern. However, in heated environments, especially those with halide (mostly chloride) compounds, and high or low pH, sulfur is an undesirable impurity. I generally prefer less than 0.01% (or 100 ppm) by mass.

Flyboy said:

“Based on the required pressure, they recommended a metal valve with nitric passivation.”

Chloride ions may undermine the oxide developed by 'nitric passivation'.

One should also be aware of any crevices in the fit up, which could lead to crevice corrosion - which may occur in the presence of chlorides, as well as contact with a dissimilar metal (galvanic coupling).

I don't believe any common stainless steels (304/316/18-8......) will matter much. Not knowing the contact time, temperature, or concentration makes this difficult.

First thought is to maybe going to a higher nickel alloy will help. Seems Carpenter 20, and one of the Hastelloy's are alloys worth consideration. My experience is HCl is very difficult to deal with and doing so will involve fairly unusual materials.

Have you considered composites, or composite (plastic, like PTFE/PFA) lined equipment? These will work, but you can get hydrogen migration and blistering through the composite coating.

ChemAir said:

I don't believe any common stainless steels (304/316/18-8......) will matter much. Not knowing the contact time, temperature, or concentration makes this difficult.

First thought is to maybe going to a higher nickel alloy will help. Seems Carpenter 20, and one of the Hastelloy's are alloys worth consideration. My experience is HCl is very difficult to deal with and doing so will involve fairly unusual materials.

Have you considered composites, or composite (plastic, like PTFE/PFA) lined equipment? These will work, but you can get hydrogen migration and blistering through the composite coating.

She’s getting the new valves from the manufacturer later this week, so I’ll see what they sent and keep you posted.