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Unknown Type of Titanium Corrosion

Unknown Type of Titanium Corrosion

(OP)
Question entails a background of NaOH production via Brine membrane electrolysis

See below figures for element internals, in a nutshell membrane(DuPONT) is sandwiched b/w anode(titanium made) & cathode(Nickel made). Anode fabrication is much more robust when compared with nickel cathode, although both best can be described as mesh structures

http://www.mediafire.com/view/t7twc952a07t6dp/2016...

ours is Generation3

http://www.mediafire.com/view/wr3ap9pxd4nkmuu/Scre...

anode/cathode chambers are sandwiched, but are segregated entirely by inbetween membrane, or otherwise H2/Cl2 mixture may cause explosion!

http://www.mediafire.com/view/iu90z6v9lqr8vqh/Scre...

stack of multiple elements is referred as electolyser

http://www.mediafire.com/view/8ruv9t22rc49hig/Scre...

http://www.mediafire.com/view/r5afqsnw1vzxf8b/2016...

Elements are connected in series with respect to applied potential, current(~13kA) is same for every element, though voltage varies depending on its health

Brine(~300g/L) is fed at anode while caustic(~30wt%) at cathode, thus both chambers are flooded with respective electrolyte ..for rest see the flash animation

http://www.mediafire.com/download/4fybm6a8vhbt1y3/...

After about 2months of dormant period & by that i mean that all of Elements of Electrolyser are drained ..there is long procedure for that. But during that period according to manufacturer ambient temperature must not get below 10celsius or membrane can get dehydrated plus you have keep them humid by flooding anolyte/catholyte headers with demin water as shown below

http://www.mediafire.com/download/iu90z6v9lqr8vqh/...

we did that all

After that dormant period we checked all the elements for membrane leakage, we refer to it as bubble test. Its fairly simple, pressurise catholyte side with ~40mbar of N2 & check whether you get any bubble at the outlet hose of anolyte. If membrane is compromised of that element ..well we see lot of bubbles. although we did found many membranes compromised but we saw something else too:

Unknown corrosion type at anode side (titanium perforated ribs, with SECRET oxide layer)

http://www.mediafire.com/view/36i95rn4m1n3znp/P_20...

http://www.mediafire.com/view/gh32aqc6eniwl1o/P_20...

http://www.mediafire.com/view/0sdj62u8x273j5f/P_20...

Compare above anode fabrication with that of cathode below. The welding is dislocated & had punctured our membranes :(

http://www.mediafire.com/download/h3w682r1n0aq691/...

Corrosion observed at anode cannot be classified as any type of corrosion as far as i know. The scorched metal points with simultaneous exposure of fresh metal isn't any type of corrosion but appears to be some sort of spark assisted deformation (Electrolyser during operation suffers ~13kA load).

Question is what do you think cause these localised (i will not refer to them as pitting) scorches. Has anyone experienced such metal blisters especially on Titanium

RE: Unknown Type of Titanium Corrosion

Very interesting, very strange. I have nothing to suggest to you but am very interested to hear what others more knowledgeable may have to suggest.

RE: Unknown Type of Titanium Corrosion

I assume you already know that titanium will burn when exposed to dry chorine vapor. This titanium may have been exposed to small amounts of chlorine during a day when the humidity was low. That could explain the scorch marks. If it had been exposed to a higher amount of chlorine it could have caused a lot more than just a scorch. Dry chlorine and titanium are highly non-compatible.

RE: Unknown Type of Titanium Corrosion

(OP)
Dry chloride existence is improbable because before draining N2 purges all gases be it Cl2 or H2. Then the system is isolated entirely. To check its isolation pressures measurements and other chemical tests are done.
So it boiled down the discussion that membrane punctures allowed caustic to travel from anolye to catholyte (its is at higher pressure than anolyte) then the caustic caused the damage but afaik titanium is resistive to even boiling caustic(cannot say anything about coating through). Mind it that while electrolyser is energized (13kA) anolyte/catholyte chambers are roughly at 88celsius. This claim was further ascertained bcz location of damaged electrode and that of membrane punctures coincides...

http://www.mediafire.com/view/2v33ooo55rri43k/147....

However this theory is revised as when we inspected even those elements which passed the bubble test (no membrane perforation), even in them anode side is damaged, the damage extent is small but the type is similar. Point to consider ..where there is damage on anode, on that position membrane is 'getting damaged' ..but not yet has developed leakages! This finding reversed what is the cause & what is the effect

http://www.mediafire.com/view/bfd2h1g50n5srj1/P_20...

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