Corrosion in hot liquor tank?

[CharlosCarlies]

Hey all,

Ran into a fun problem yesterday morning that has us pretty stumped. Our head brewer noticed a small leak on the bottom of our hot liquor tank that after later inspection is almost surely steam condensate. After talking to our tank manufacturer, they're of the opinion that a leak on the steam jacket side is almost impossible to (cost effectively) fix despite the tank being barely 3 years old.

To add even more fun to the ordeal, after checking inside the tank, there's now a pretty significant amount of corrosion on the walls. I'm rarely involved in the day-to-day brewing process anymore, but my head brewer seems fairly certain that there wasn't any noticeable corrosion at least since the last time we dosed up the tank about a week or two ago.

Only thing we've really changed recently that I can think of was dosing significantly more calcium chloride into the hot liquor (bringing us from ~50 ppm to ~150). I know stainless and chloride in general don't mix well, but simply adding more calcium chloride couldn't possibly have made that big of a difference so quickly could it?

Also it's possibly just complete coincidence, but does it seem odd that the steam jacket failed around the exact same time we noticed sudden corrosion in the tank? Is it possible something was just seriously up with our city water? I'm at a complete loss here!

[dick murton]

Definitely the CaCl2. This solution is quite acidic, and there will be free chloride ions, which rapidly attack the stainless, especially since it is hot, and reactivity is greatly increased. Add CaCl2 as solid directly to the grist / mash, or if adding as solution, directly to the mash mixer through polythene or similar (non metallic) pipe. If you want to add it to the sparge, then consider sprinkling powder on top of the mash just before sparging starts.

Welds are renowned for creating stress areas, which are more prone to corrosion by chloride form liquor treatment salts, of hypochlorite (or iodide ions in the case of iodophor sterilants).

[CharlosCarlies]

Thanks Dick...the calcium chloride was the only thing I could think of. Is going from 50-150 ppm really enough to create an environment where corrosion that we haven't seen in 3+ years of business can happen in less than 3 weeks? If so, damn.

Only other thing that's been bugging me, is why would a weld in the steam jacket be the one to fail? Unless the city drastically changed our water source, the boiler feed water shouldn't have changed at all.

...which suddenly leads me to another possibility. Can't believe I didn't think of this: We've been softening our boiler feed water with salts, of which variety I'm not currently sure. I'm guessing the chlorides from said salts could also be causing problems, no?

Wow, the longer you own a brewery and the more you learn, the more you realize you don't know shit!

[NHBrewer23]

The free chlorides are most likely the culprit. If the tank is fabricated from 304l stainless, as little as 10ppm of free chlorides will corrode the tank when the water is hot. 316 stainless can handle up to 50ppm of free chlorides at elevated temperatures. Seeing that the corrosion is occuring on the steam jacket, it makes sense that the issue is chloride related. You may be able to consult with a company who can pickle-passivate the tank on-site and patch any leaks if the corrosion isn't too deep.

[CharlosCarlies]

Thanks guys...it all seems so obvious now. Ughh.

Any thoughts on the boiler feed water being softened with salts? Should we have gone with something like RO instead?

Cheers and thanks again for all the feedback.

[dick murton]

I would have thought you water softening / demin plant should not be causing the problem as (a) these are common process for preparation of boiler water, and (b) unless the steam has got some water carryover, it will be free of mineral salts, or have such minimal amounts that this is not a problem.

Re "why the welds?" - the welds are the areas which have been subjected to maximum stress due to the high temperatures attained during welding. I was told once, but have never had this confirmed, that the metal grains in this area tend to be coarser and without being passivated properly, don't have such a good CrO2 layer, and are thus more susceptible to corrosion.

Wow, the longer you own a brewery and the more you learn, the more you realize you don't know shit! This comment got me thinking as well. In a couple of old breweries, we also used to add calcium chloride and gypsum to the hot liquor tanks. It dawned on me seeing your comment that we got away with it because the breweries were old, and these were cast iron tanks, or being fed through copper pipes to the mash tuns etc, not lovely shiny stainless. It also made me think how close I have been to advising people that they can add CaCl2 to the hot or cold liquor tanks - and if it had been added to the cold liquor tanks how dramatic the effect on the wort chiller would have been.

So a big thankyou for waking me up on this point.

Cheers

[ScottD50]

Dick,
So your comments to this thread have me wondering about my own problem on another thread. We are 6 months old and have been adding calcium chloride and gypsum to the HLT. I now have a brown stain on the inside of my HLT and I'm lead to think that it was these additions that may have caused this staining due to your comments about not adding these directly to HLT. Is this possible? If so-do you have an idea how to remove the stain? i am not adding these additions to HLT anymore.

thanks
Scott

[thelorax121]

Just another data point: the crack in the steam jacket can also be a manufacturing flaw unrelated to the chlorination effects. I have seen some manufacturers who have used mild steel internal support beams and/or jacketing that quickly contaminates the stainless it is welded to, causing it to pit and fail quite quickly. One brewery I know of had pinhole leaks in thier brewhouse in under a year from this effect....

[CharlosCarlies]

While we're still attempting to piece together this puzzle, glad to see it started a decent discussion even if it's at our expense! Thanks again for all the feedback so far.

[dick murton]

I would rig up a temporary sprayball if necessary and then recirculate 5% nitric acid / phosphoric acid at about 60 C, for an hour, to remove any hard water scale and passivate the stainless. I know there are all sorts of threads and advice about using citric or tartaric acids, but since I'm not sure how good these are at removing any hard water scale, so I am going to suggest the method I know has worked for me in the past. If you can't do it hot, then I still suggest a cold descale with circa 2 % phosphoric / nitric mix, and then perhaps consider using tartaric or citric acids for final passivation.

If you look at the Murphy & Sons (Nottingham, UK) website you will find addition rates for CaCl2 and CaSO4 to the grist prior to mashing in , and just prior to sparging (see the tech data sheets etc). These rates differ somewhat from the rates you would need to adjust the liquor composition, but give results leaving nominated mineral ion levels. This means you have hugely reduced risk of chloride ion corrosion.