Peteru,
This is the neck weld of one of mine (Beerkat) and as you can see it is not bad. I did look at the internals (prior to perc clean, Starsan and filling with brew) and couldn't see any discolouration or weld marks that were visible, but most of the welds would not be visible without a mini camera on a flexi-cable.
wide eyed and legless said:
I am sure that on either the Beerkat website or somewhere in the Beerkat thread it was mentioned that the kegs had been passivated but if peteru had laid his down then the weld around the neck may not have been, though I would imagine that they would all be submerged in the acid bath.
Further reading about the metallic taste and solutions is to do the passivating at home before using a new keg, or as the keg starts to produce the metallic taste. It has been covered on this site before a couple of years ago and John Palmer covers it in his, 'How to Brew' book.
http://www.beerandloafing.org/hbd/fetch.php?id=52083
Yes one would expect passivization to have occurred to some degree, otherwise they would look bloody rough. It is the thoroughness of post weld clean and 'pickling' (acid bath) of the internal surfaces before passivization occurs that is in question with my hypothesis.
I can seen from the outside of my kegs that passivization did occur, but when one looks as Peteru's neck weld and some of the clean up on mine below one can see that even on the externals that have been ground down and acid treated, there are some interesting welds (Peteru's) and black chromium carbide marks left that were not cleaned with grinding and acid wash properly so therefore passivization did not occur for those spots.
This is one of my two mini-kegs showing a black weld mark in a divot (I assume this is chromium carbide from the weld). It is a sign that the acid pickling did not clean that divot. This is external, so if it is similar on the internal then that black carbide will leach into my beer or when I treat it with acid (one would expect if the outside was missed, then the inside may well be the same or worse). There is also some discolouration from heat around some of the spot welds that were not removed by pickling. Nothing terrible, but worrying that the inside might not have been checked at all.
From top
From side, same divot continues down side.
Areas on the outside were missed, so I then question whether the inside was done thoroughly or not. A black film isn't a common indicator for corrosion of SS, so it must have come from somewhere. As chromium carbide can be caused by welding and is black and would be disolved by acidic solutions (beer included) it is a likely candidate. There have been a few who have posted about black films after Acid use or beer being left in the keg for a while, so this also fits chromium carbide as a candidate. A metallic taste would also fit it as a candidate. The only thing I could conceivably think of was that some of the welding produced intergranular corrosion, which if you read the links to my early posts indicates that if not removed cleanly, can cause structural integrity issues with the steel with further corrosion (worse than pitting corrosion). Given these are pressurized vessels then if that corrosion continues over time then this could conceivably be an issue.
You can passivate those welds all you want, but when crevices and ridges are present in stainless, then future corrosion is invited, just like scratches and crevices hide bacteria in a plastic fermenter, scratches and crevices in SS invite corrosive substances to lodge, despite rinsing and they can remove the protective layer and cause pitting corrosion.
In short, if the SS is not shiny, it will corrode. If it is not smooth, then far more care needs to be taken when rinsing out. This can be passivized at home as you say, but when the potential corrosion issue is out of vision, how often should it be passivized and what damage has been done prior. To a pot or non-pressurized vessel this is not such a worry and can be lived with. To a certain extent so too could it be fine in this case, but there may well be a batch or a particular welder at the factory that can't do their job properly, which should be identified.
I for one will be checking both of my mini-kegs' internal surfaces with a small mirror and flash light as soon as they are empty of beer.
