Corrosion – Part Three

This is the third in a series of blogs on corrosion and corrosion resistance in our stock grades. The first two blogs (found on our web site) looked at types of corrosion and the steps we take to minimise it, while the second looked at then the factors (particularly chemistry) that affect a metals’ ability to withstand corrosion. This blog will discuss the tests for corrosion and a quick way of ranking materials resistant against pitting corrosion while the final blog in the series how our stock grades rank in terms of corrosion resistance.

Obviously if a metal is going to be used because it should have resistance to a particular type of corrosion, we need to check that the material as produced meets that requirement. To enable this a range of standard tests has been developed to test material.

The most frequently used are as follows, but do not worry if you are still a bit confused, our Process Department can guide and advise on what is needed to meet a specification or a requirement:

Intergranular Corrosion Testing – ASTM A262 Practice A, B, C, D, E – the standard practices for detecting austenitic stainless steel’s susceptibility to Intergranular Attack. Used to test Alloy 825 (test C, after a sensitizing heat treatment and exposed for 240 hours).

Intergranular Corrosion Testing – ASTM G-28 Methods A (ferric sulphate) & B (cupric chloride) – the standard practices for detecting wrought, nickel-rich, and chromium-bearing alloys’ susceptibility to Intergranular Attack. Used to test C276 (A & B – 24 hours exposure ) and Alloy 625 (A – 120 hours exposure) materials.

Corrosion Resistance Testing (ASTM G48 Methods A & B) – the standard test method to identify and measure Pitting and Crevice Corrosion Resistance of Stainless Steels and related alloys by the use of a Ferric Chloride Solution.

Corrosion Testing – ASTM A923 Method C – the standard Test Methods for detecting detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels.

Salt Spray/Salt Fog – ASTM B117 – the standard practice for Salt Spray (Fog) testing designed to provide corrosion resistance information on metals and coated metals, particularly in marine environments.

Humidity & Temperature Testing – determines how components, subsystems and complete systems behave in severe environments that involve elevated temperatures and high or fluctuating relative humidity, particularly subject to climatic changes.

Passivation Testing – one of ASTM A380 – Practice for Cleaning, Descaling and Passivating of Stainless Steel Parts, Equipment and Systems, ASTM A967 – Specification for Chemical Passivation Treatments for Stainless Steel Parts (based on US Defence Department standard QQ-P-35C) or BS EN 2516 – Passivation of Corrosion Resisting Steels and Decontamination of Nickel Base Alloys. Stainless steels “self-passivate” i.e. create a monomolecular oxide surface film to lose its chemical reactivity. The test is used to confirm that the metals’ ability to do this is intact.

Finally a note on PREN Numbers – “Pitting Resistance Equivalent Number”.

The PREN is a theoretical way of comparing stainless steels, using their chemical compositions. The PREN (or PRE) numbers are useful for ranking and comparing the different grades, but should not on their own be used to predict whether a particular grade will be suitable for a given application where pitting corrosion may be a hazard. PREN numbers in particular downplay the alloying effect of Nickel in pitting resistance. The most commonly used version of the formula is PREN = Cr+1.5x(Mo+W+Nb)+30xN, and the values below have been calculated using this formula. However there are several modified formulas and it would be worthwhile checking on the calculation used (especially what chemical elements are included and their weighting) before accepting the PREN numbers at face value.

Commercially available steels in grades 1.4410, 1.4501 and 1.4507 are often stipulated to have actual PREN values over 40, although grades with a PREN of 40 or more are known as ‘super’ austenitic or ‘super’ duplex types as appropriate.

Below are the PREN (based on nominal chemical values) for some of our stock grades, ranked according to the calculated PREN:

Remember the below refers to the resistance to pitting corrosion only – our next blog will widen the scope and look at how resistant our metals are to all forms of corrosion. In the meantime, if you have any questions please contact our Special Process department on ++44 114 232 9241.

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