linglis

Topic: Effects of Sulphuric Acid Anodizing of Titanium

Hi all, I have a query and was hoping someone might be able to offer some advice.

I work for a check valve manufacture and we are supplying some small Titanium (B348 Grade 2) valves where they will be used in a PTA plant where corrosion is a big issue. There is a restriction placed on the chemical analysis of the titanium where by the Hydrogen content must be <60PPM. The material we are supplying is certified as such.

The Titanium valve bodies have been finished by Sulphuric Acid Anodizing which has raised the following query from our customer:

"The anodizing procedure adopted (using sulphuric acid) has the potential to introduce hydrogen into the titanium which, at concentrations above 60ppm, causes severe corrosion of the titanium. Please confirm that the hydrogen in the titanium after anodisation is < 60 ppm."

Can anyone offer any advise on this query or know of any non-destructive method of analysing the hydrogen content of the anodized surface of the valve in order to confirm compliance with the hydrogen content restriction.

Any help would be most appreciated.

Thanks,

Lee Inglis.

AluConsult

Re: Effects of Sulphuric Acid Anodizing of Titanium

Hey Lee Inglis

Anodizing of Titanium in Sulfuric Acid leads to hydrogen embrittlement cause by the absorption of hydrogen over time. This is also observed when using titanium as a rack material for anodizing aluminum.

Hydrogen can be removed from titanium by heating at 1200-1500 F in vacuum. However, if cracks have appeared hydrogen embrittlement damage is not reversible. If the metal has not yet started to crack, the condition can be reversed by removing the hydrogen source and causing the hydrogen within the metal to diffuse out, possibly at elevated temperatures. Susceptible alloys, after chemical or electrochemical treatments where hydrogen is produced, are often subjected to heat treatment in order to remove absorbed hydrogen. There is a 4-hour time limit for baking out entrapped hydrogen after acid treating the parts. This is the time between the end of acid exposure and the beginning of the heating cycle in the baking furnace. This per SAE AMS 2759/9 Section 3.3.3.1 which calls out the correct procedure for eliminating entrapped hydrogen (ref. wikipidia).

There are two ASTM standards for testing embrittlement due to hydrogen gas. The standardASTM F1459-06 Standard Test Method for Determination of the Susceptibility of Metallic Materials to Hydrogen Gas Embrittlement (HGE)|Test] uses a diaphragm loaded with a differential pressure. The test ASTM G142-98(2004) Standard Test Method for Determination of Susceptibility of Metals to Embrittlement in Hydrogen Containing Environments at High Pressure, High Temperature, or Both uses a cylindrical tensile specimen tested into an enclosure pressurized with hydrogen or helium (ref. also wikipidia).

It is not the hydrogen in the coating but in the material you should try to establish.

Why do you anodized the titanium valves? Titanium are inert in a lot of solutions. Sunny regards

Anne, www.anodizingworld.com

AluConsult's Website

linglis

Re: Effects of Sulphuric Acid Anodizing of Titanium

Hi Anne,


Thank you very much for the reply.  I've had little luck elsewhere with question and your response has been most positive.

Hydrogen embrittlement is the reason this question is being asked. 

We haven't manufactured these valves directly but have bought them in from another supplier as part of a package we are selling to a project.  The valves arrived in the anodized condition as standard from our supplier. This is not for aethestics but is to aide the corrosion resistance of the material by protecting the surface from "free iron" contamination and other airbourne contaminants that could affect the material after it has had been cleaned and ferroxyl tested. 

The titanium in question has a certified Hydrogen content of less than 60ppm and our customer has asked if the use of sulphuric acid (which obviously contains hydrogen atoms) could increase the hydrogen content of the titanium at the surface beyond the allowable 60ppm and therefore putting the material at a higher risk of succumbing to hydrogen embrittlement.

I don't think that this should be an issue but I do have to justify this somehow to our customer before they will accept the valves.  I was planning on having carrying out test with 2 pieces of titanium from the same melt.  I would have one test piece anaodized (the same as the valves) and leave the other piece unfiinished and then have both pieces analysed for hydrogen content and compare the results.  Not sure how else to resolve this one really.

Thanks again,

Lee Inglis