Selection of stainless steels for handling phosphoric acid (H3PO4)

Introduction

Phosphoric Acid is also know as orthophosphoric acid and is classed as a weak acid.
Austenitic stainless steels have good corrosion resistance to chemically pure phosphoric acid.
Wet process phosphoric acid (WPA) can be aggressive.

It is used as a chemical-cleaning agent for stainless steels but is not considered to be a 'passivating' acid.

Commercially concentrated acid is around 85wt. %

Corrosion resistance of stainless steels

The austenitic stainless steels have good corrosion resistance to chemically pure phosphoric acid over a wide range of concentration and temperature.
The iso-corrosion diagram 0.1mm/year lines are represented for the 304 (blue) and 316 (red) types and show that the 304 types should be satisfactory up to acid's boiling point to around 25% concentration.
(The broken line represents the boiling point)

 

 

 

 See Corrosion Handboook for source data.

At higher concentrations 316 is resistant at higher temperatures for any particular concentration i.e. the lines are essentially parallel.
The 316 types should be considered if chlorides are likely to be in the acid.

Wet process phosphoric acid (WPA) can be aggressive towards stainless steels, depending on the range of impurities that the acid contains. See Corrosion Handbook for specific data on WPA.
This can be of particular concern in bulk handling and transportation of raw phosphoric acid and specialist advice is needed to optimise grade selection.

Chlorides, fluorides and sulphuric acid impurities increase the risk of corrosion, along with increases in temperature.
The more pitting resistant steel grades should be considered when these impurities are known to be present.

Uses for phosphoric acid with stainless steel

Phosphoric acid is used as a chemical-cleaning agent for stainless steels.
It is used in commercially available stainless steel cleaning preparations and so if used in accordance with the manufacturers / suppliers instructions will not etch or corrode the steel surface.
Phosphoric acid is not considered to be a 'passivating' acid but the clean surface left after treatment should allow the stainless steel to naturally self passivate.

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