Corrosion Resistant Coating for Steel Tanks

Shhh. Please do not use all-caps.

Epoxy paint might be a suitable choice.

Consider consulting a paint supplier.

I am not a paint expert, but I suspect that epoxy could work well in this case, as I already said. I don't plan on saying it again in private messages. If that seems caustic, it is probably from the soda pop accompanying my rum. I hope this exercise in informal chemistry is not too mysterious.

Thanks, Mr. Tonado.

The epoxy paint applied previously has started showing signs of corrosion.

The solution is Caustic Soda 50% concentration in the mild steel tank.

ashokjp42

Do you mean 50% normality or 50% w/w? 50% normality is only 20 g/l, ~ 2%.

I haven't got the data sheet to hand but if I remember right steel caustic tanks do not need lining. Caustic embrittlement is a problem so tanks must be stress-relieved after fabrication. You should be able to get COSHH and data sheets from the caustic supplier.

Found my caustic data. Confirms tanks not usually lined. If lining used it would have to be very good stuff as if it started to break up it would cause a lot more problems in pumps etc than a small amount of iron contamination.

Caustic embrittlement is a less of a problem than I said. As long as the liquor is not > 50°C the tank itself does not need to be stress-relieved. But 50% w/w caustic freezes at 7°C so you'll likely need a heater, and you have to ensure this doesn't cause local heating > 50°C.

Thanks, Codemaster.

I will tell d customer to try plastic liner.

ashokjp42

Modern caustic soda facilities often use plastic tanks; PVC is widely used as the wetted material and tank manufacturers can be found to manufacture suitable tanks in most sizes.

As others have said, my advice is to change to a tank made out of the PROPER material rather than buying a cheap steel tank and then trying to figure out how to apply a protective coat that can withstand the corrosive effects of the material it contains.

Your fooling yourself if you think this will save money (and maintenance time) in the long run. Buy the proper tank.

There's nothing wrong with using a steel tank for caustic, steel is just as PROPER as plastic, and it's been used for years. There's no reason I'm aware of to think plastic is cheaper in the long run. If OP wants to do a life cycle cost on both, he can, but he only asked for advice on lining a steel tank.

My guess is steel will be cheaper for a big tank, say 5 m³ or more, maybe plastic for a small one, but we don't know what volume he has in mind.

There's nothing wrong with using a steel tank for caustic, steel is just as PROPER as plastic

Yes, but I didn't think it could be any old grade of steel (especially if the caustic solution was hot).

I was working on the assumption that the existing steel wasn't suitable for the application hence the request for an additional coating by the OP to be added to an existing unsuitable grade steel tank.

OK, you could be right. My data just refers to carbon steel. There are quite a few grades of carbon steel, but I doubt if they differ much in resistance to caustic. (it says plastic is OK as well, but doesn't say any particular one).

Maybe OP will come back with more but I won't hold my breath.

Found this at Cole Parmer

There was no data for aqueous solutions under 20 wt.%.

Maybe he would be OK with a 50% normal (2wt.%) solution, but I wouldn't count on it.

Chemical Compatibility Database from Cole-Parmer

Material Selected: Carbon Steel

Interesting. Most of my corrosion data is for rubbers and plastics, all I have on steel is an old issue of Perry. But according to that it's noway near as bad as D-severe effect implies. Up to about 100°F and 50% it says 2nd from best category, between 0.002 and 0.02 in/year.

So I stand partially corrected, maybe plastic is a better selection nowadays. But I still think steel is an option, or it wouldn't be recommended in the caustic data sheets. Steel tanks must have a working life 10 years at the very least or they wouldn't get a look-in.

Thanks, Mikerho.

The chart guides to go in for PVC , UPVC liner or Ceramic.

ashokjp42

<...hot...>

Exactly. Without knowing the temperature it is impossible to give meaningful suggestions.

Coatings are only as good as the surface preparation done prior to applying them.

I do not recommend any coating be applied to a used tank.

PVC, PPRO, stainless steel, titanium will all give good results.

Carbon steel will not.

Maybe line your tanks with welded PVC liners with Ti hardware for good results.

Chemical Compatibility Database from Cole-Parmer

Material Selected: Epoxy

Paint will not fulfill this function.

You don't specify whether you are referring to new equipment or the repair of an existing system but in either case, when it comes to water treatment, you need qualified water treatment engineering services.

One has to wonder why inappropriate materials would be chosen for a water treatment plant to begin with.

The only conclusion I can reach is that the builders of the plant are trying to save money by using inferior materials.

If you are looking for a CHEAP, temporary solution, epoxy paint may be it.

Any kind of epoxy will fail at some point in the future. Same with paint. Any slightest crack or chip in the lining will allow the caustic to seep behind the coating and attack the steel.

Glass lining won't work because the caustic etches the glass. Ever wonder why acidic reactions are sometimes done in glass lined reactors but caustic aren't? Incompatibility! For caustic reactions St Stl is usually used.

About 70 (?) years ago a few steel reactors were made with a thin st stl liner bonded to the steel surface on the side with the contact with the caustic. These were most often used for blending of powders but some were made for liquids. This method would now be very expensive, more than an all st stl unit.

Unless the quantity to be stored exceeds the available capacity that can be built, most chemical plants now use welded polypropylene or other similar compatible plastic storage tanks for caustic storage. In general they can also be made to your custom requirements.

Go with plastic, that's what we used for it's storage in our waste water treatment plants. Worked out better than planned. They are also generally less expensive.

Good Luck, Old Salt

<...drinking water...>

The original poster doesn't state whether this is industrial or whether it is municipal water treatment. The original poster doesn't state whether this is incoming water or outgoing effluent.

The application is fundamental to the original poster's selection of equipment, and that information has been denied to the forum. That's the way it is on CR4, all too often, sadly.

Epoxy paint is not recommended for potable water. OP has stated the water tank is for water treatment plant. May be for industrial application only.

Epoxy painting is correct solution. The process of painting needs to be followed. Surface should be cleaned and recommended primer should be applied in number of coats as suggested. After primer paint has dried epoxy paint should be applied.

At what temperature do you intend to maintain the caustic soda ?

Temperatures > 140 F eliminate any plastic or FRP option,..... IMHO ....

Do you intend to use an internal tank heater ? If so, what type ?

This will also affect the type of tank and possible coating selection.

An external tank heater in a circulation loop for 50% NaOH may be a good choice

Plasite has, for many years offered quality successful coatings specifically for 50% NaOH steel tanks.

They were bought out by Carboline some time ago ....

Hi
Check SODIUM HIDROXIDE 50% Is the same Caustic soda 50%
http://www.coleparmer.com/Chemical-Resistance
you wellcome
WP

As <...caustic soda...> is a good paint stripper, especially when hot, the solutions to the particular problem might be few and far between.

No-one on CR4 is liable if this goes wrong, by the way.