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Engineering Fields - Chemical Engineering - New Member

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Unknown Type of Titanium Corrosion

03/07/2016 4:51 PM

Question entails a background of NaOH production via Brine membrane electrolysis

See below figures for element internals, in a nutshell membrane 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/2016-03-07--1457365430_179x63_scrot.png

ours is Generation3

http://www.mediafire.com/view/wr3ap9pxd4nkmuu/Screenshot%20-%20070316%20-%20205500.png

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/Screenshot%20-%20070316%20-%20205623.png

stack of multiple elements is referred as electolyser

http://www.mediafire.com/view/8ruv9t22rc49hig/Screenshot%20-%20070316%20-%20205837.png

http://www.mediafire.com/view/r5afqsnw1vzxf8b/2016-03-08--1457385978_736x416_scrot.png

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/membrane_cell.swf

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/Screenshot_-_070316_-_205623.png

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_20160307_151349.jpg

http://www.mediafire.com/view/gh32aqc6eniwl1o/P_20160307_151427.jpg

http://www.mediafire.com/view/0sdj62u8x273j5f/P_20160307_151437.jpg

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

http://www.mediafire.com/download/h3w682r1n0aq691/P_20160307_151817.jpg

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

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#1

Re: Unknown Titanium corrosion type

03/07/2016 4:59 PM

Have it analyzed and hire a consultant.

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#2

Re: Unknown Type of Titanium Corrosion

03/07/2016 8:51 PM

I recommend you join NACE as there are people in that organization who thrive on this sort of thing. They are an international organization with societies all over the world.

I'm sorry I cannot offer any more as this is well outside my areas of expertise.

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#3

Re: Unknown Type of Titanium Corrosion

03/08/2016 12:38 AM

I don't profess to know anything about your process, but Titanium is particularly susceptible to Hydriding - ie the absorption of Hydrogen - which takes place more readily in high pressure applications (especially anhydrous ones), low (below 3) or high (above 12) pH environments, galvanic couples (impressed voltage greater than -0.7v as compared to silver/silver chloride cell), any or all of which as may be present in your application. I would expect that the mere presence of Nickel in the mix would be providing a galvanic couple of sorts.

Is it possible that your demineralised water is not as demineralised as it should be? Any addition of Fe to the mix could cause problems as it may accumulate on the anode and thus puncture the membrane.

As I said at the beginning, I know SFA about your process, so take the above as conjecture only.

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