Titanium & 50 - 500 mM NaOH compatibilty 1

[usjbh]

How well does Titanium Gr 2 drawn capillary tubing, Gr 4 & 6Al4V alloy machined pump components perform with 50 - 500 mM Sodium Hydroxide to 40 degrees Celcius at pressures up to 5,000 psig? We have done some static compatability testing with 100 mM Potassium Hydoxide and extended life testing with 1M NaCl + 50mM Sodium Borate to pH 9.3 and did not seen any evidence of compatibilty problems using these mobile phases but have not yet undertaken tests using the higher NaOH pH.

 

[unclesyd]

I take it that "mM" is moles/million moles?
My data shows no problems with Grade 2 if the temperature is below 70°C and Ph is less than 12.

I have no corrosion data for Grade 4 or 6Al4V in sodium hydroxide. I would probably watch the 6Al4V material closely.

 

[TVP]

In general, Ti alloys are very resistant to alkaline media. At concentrations below 50% and temperatures less than the boiling points, Ti is quite resistant to NaOH and KOH, with corrosion rates below 0.1 mm/year. At pH ~ 9 and relatively low temperature of 40 C, both of these alloys should provide good performance.

 

[usjbh]

My thanks to for the response and vote of confidence for my application. I've delayed responding pending further investigation into two other aspects of performance, passivation and hydrogen embrittlement.

GR 4 & Gr 5 (6AL - 4V) Titanium were identified as more susceptible to SCC (Stress Crack Corrosion) in the presence of hydrogen. NaOH and high pH set off the alarm bells with the metallurgists on this point.

Additional recommendations outside this forum were to switch from Gr 4 to Gr 2 and from Gr 5 to Gr 23 (6AL-4V ELI) to reduce the SCC risk.

In addition to HNO3 passivation of our machined parts which we were doing, thermal (~1000 F) passivation in air was recommended to build up a heavy Rutile protective oxide film versus the Anatase open structure that forms at lower temperatures and would not provide as effective a barrier to hydrogen.

Timet has a pretty good guide "Corrosion Resistance of Titanium" that, based on Laboratory experiments, lists three conditions that have to usually exist for hydriding to occur.
1. For pH <3 or >12 the metal surface must be damaged by abrasion or impressed potentials more negative than -0.70V.
2. Hydriding failures below 77 C are rarely encountered unless there is severe tensile stress.
3. There has to be a hydrogen generating mechanism e.g. galvanic couple, cathodic impressed current or intense dynamic abrasion to dpress the metal potential below that required for spontaneous evolution of hydrogen. They further note that hydriding can be avoided by altering at least one of the three conditions. My application does not have any of these three conditions present. Additionally, anhydous chemicals are not used and the solutions are never in a state where elemental hydrogen gas generation takes place.

I will pursue the additional Rutile passivation step, seek chanegover to Gr 23 alloy from Gr 5 but for purposes of machinability and tensile strength requirements that require Gr 4, will not be able to switch to Gr 2. It looks like Gr 2 for the tubing looks O.K. It also turns out that we did previous static exposure tests of Gr 2 & 4 passivated samples with 1M NaOH and found no problems.
The high pH NaOH enviroment now looks to be lower risk than first thought.