Will a passive layer form on stainless-steel in pure (0.055 μS/cm) water with an oxygen concentration < 50 ppb. If so, why is this type of water generally called 'deoxygenated'? If a passive layer (chromium oxide) does not form with such low oxygen concentration why is this, since there IS still oxygen in the water.
$\begingroup$ There are too many unknowns for a definitive answer. For instance there are many kinds of stainless steel. In general water at room temperature wouldn't "attack" stainless steel. In steam plants however steam under high pressure is much more reactive. Having oxygen in the steam makes the oxygen/water combination even more reactive. Since there is oxygen in the air it is nearly impossible to get ALL the oxygen out of any water sample. Look at it this way. Air has about 78% nitrogen and 21% oxygen by volume. A mixture of 99% nitrogen and 1% oxygen would kill you. $\endgroup$– MaxWFeb 13, 2017 at 19:18
From a practical standpoint, once the stainless steel item is made the passive layer immediately begins to form. Passivation with acids is a process used to accelerate this process and have parts ready to use in a shorter period of time. So if you have any high quality SS parts, they will come to you with the passive layer already developed.
Theoretically speaking, if a completely naked section of stainless was put into this "pure water", there would still be some oxide formation because the specification of <50 ppb still allows for some oxygen to be present. However, this should not have any practical implications. I suspect the name "deoxygenated water" is used to indicate there was a conscious process used to reach such a low amount of free oxygen in the water but not to claim o% free oxygen content.