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Stainless Steel Etch formulation for Penetrant Inspection?

RFQ: Before performing "Liquid Penetrant Inspection" on stainless steel, a local etching should be performed. I'm looking if paste or gel etchant exist and from where I can purchase it?

Thank you,

Said Ziane
aerospace - Montreal
2006

Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)

Q. We are a metal finishing/NDT facility that does pre-penetrant inspection etching. Currently, we process stainless steel parts in an unheated nitric/hydrofluoric acid mixture. Due to the long processing times necessary (especially 300-series austenite grades, often on the order of 5-10 hours to remove 0.2-0.4 mil), we are investigating the installation of a heated tank.

One formula that I've come across is similar to the nitric/hydrofluoric tank that we currently have, only heated to 120-140 °F. Another formula uses the same amount of nitric acid (and half the concentration of HF), but also requires an equivalent amount of phosphoric acid, as well as a small amount of a wetting agent, again heated to 120-140 °F.

What is the function of the phosphoric acid in the latter formula? How does the addition of a wetting agent improve the function of the etch tank? Under what conditions would the addition of these components be advisable?

Thank you for your advice.

Joshua Meisner
Process Engineer - West Babylon, New York
August 6, 2012

A. The surfactant added to the etching solution will assist in the removal of metals and metal oxides from the surface by allowing easier chemical interaction with the surface or reducing interactions from adsorbed compounds, like organics.

The phosphoric acid increases the solubility of iron or the dissolution of iron compounds. Often, a combination of acids becomes synergistic with the combination working better than either compound alone. Daryl.

Daryl L. Roll
Costa Mesa, California, USA
August 21, 2012

Thank you, that makes sense.

Joshua Meisner [returning]
Process Engineer - West Babylon, New York
August 30, 2012

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