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pH and acid measurement in stainless steel pickling

Q. We are looking pickle duplex stainless steel with a HN03 and HF mix.

I'm not a chemist! But, I am confused.

We know the acid is about 26% wt HN03 and 7% wt HF delivered to us. We then mix that 2 to 1 with water. This does not change the pH but does make the pickle less aggressive. I not sure I understand this as I thought pH is an H concentration factor.

How can we calculate the percentage of Nitric and hydrofluoric in our process. I suspect a simple titration method could be used.

Is it common practice to top up pickling baths with stronger liquors to keep the concentrations correct?

Any pointers would be very much appreciated.

Scott Wilkinson
Engineer - Wakefield, England
January 27, 2010

February 2, 2010

To answer at least some of your questions:
pH is a log scale and therefore at the extremes of acidity and alkalinity even large changes in concentration result in slight changes in pH. I wouldn't expect diluting such a relatively high concentration to have a massive effect on pH.

It will be possible to check with titration methods however a mix of acids can be problematic. One more common method with be to perform a total acid titration and then perform a test for fluoride to check the HF concentration. You would find the nitric acid concentration by subtracting the HF from the total acid. You will need to look into ion selective electrodes for fluoride.

Finally, yes it has always been common practice wherever I have worked to maintain concentrations by adding more concentrated solutions.

Ciaron Murphy
aerospace - UK

Scott,

pH measurements are really only appropriate for quite dilute acid solutions. Once you get up to the concentrations you are talking about pH becomes all but meaningless.

A standard acid-base titration would be the best control method for solution.

Use 1.0N NaOH [1N NaOH on Amazon] and methyl orange [on eBay or Amazon] indicator. You can find the method in most text books about plating such as the Canning handbook and the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] as well as books on analysis techniques (my favoured volume is Vogel's Textbook of Quantitative and Semi-Quantitative Analysis).

Brian Terry
Aerospace - Yeovil, Somerset, UK
February 2, 2010

pH is used to measure weakly acidic or alkaline solutions. Your etch is way too strong to use pH as a control. Apart from this the HF will dissolve the electrode and make it useless for any measurements. Whatever numbers the meter was giving, they are purely fictitious and the electrode is now useless.

Titration is the usual way to control the etch but it is not a simple one and you will need need a little training.

I am sure that you are aware that HF is particularly dangerous. It requires that the operators have training and specific first aid measures immediately available. You will also, of course, have a COSHH assessment in place that will have considered this.

Geoff Smith
Hampshire, England
February 2, 2010

Titration equipment manufacturers have developed a method for analyzing HNO3, HF and Fe in stainless steel pickle liquors. Using thermometric titration, it is possible to differentiate endpoints for HNO3, HF and Fe. The method is fast, and is amenable to the full range of automation possibilities. The titrations are conducted in plastic vessels without special insulation.
Bets regards,

Tom Smith
- Brisbane, Queensland, Australia
September 20, 2011

Q. We are regularly pickling stainless steel filtration pipes using a mixture of Nitric Acid and HCl. When we start to notice the HAZ is not cleaning properly we add more of the 2 acids to increase the concentration.
Recently we've had some failures in the field which are related to the pickling and passivation so our question is what methods are there for testing:
1) Acid levels
2) Free Iron levels
in the pickling solution?

We appreciate any direction you can provide.

Kirk Riddell and Dennis James
- Potaram, Ratchaburi, Thailand
October 9, 2019

October 15, 2019

A. Dennis,
I hope you meant to say a mix of nitric and HF. Never use HCl on stainless steel.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

Thanks! Yes. You are correct, I got the acids mixed up

Kirk Riddell and Dennis James [returning]
- Banpong, Ratchaburi, Thailand
November 4, 2019

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