Corrosion behaviour of stainless steel long products with different delivery conditions (first part)

Stainless steels are characterised by different chemical composition and microstructure. These parameters, as well as the surface finish of the product, influence corrosion resistance. Centro Inox has recently carried out an investigation aimed at estimating the different corrosion behavior of stainless steel long products (bars), related to the materials and delivery conditions most frequently marketed.

The aim of this investigation was to define the different corrosion resistance of five classes of stainless steel (AISI 430, AISI 430F, AISI 303, AISI 304 and AISI 316) with the respective P.R.E.N. index (Pitting Resistance Equivalent Number i.e. an index that relate the stainless steel’s chemical composition to its relative pitting resistance) and, at the same time, of two different delivery conditions that are usually required for long products (ground finish and drawn finish). For this purpose, the following tests were conducted

  • Potentiodynamic tests
  • Potentiostatic tests
  • Immersion tests in FeCl33
  • Atmospheric exposure tests
  • Salt spray tests

POTENTIODYNAMIC TESTS – They are accelerated tests useful to quickly characterize the localized corrosion behaviour of metals. The final results, expressed as pitting potential (Epit) can be used to perform a corrosion resistance ranking for the different grades and finishes analysed: the higher the measured Epit, the higher the corrosion resistance.
In this case, three-electrode cell tests were carried out, using a stainless steel sample, a reference electrode in Ag/AgCl and an activated titanium counter-electrode. During the test the potential of the stainless steel sample was increased from the free corrosion value to the localized pitting corrosion conditions, with 1 V/h scanning speed. For each steel and for each finish, tests in solution with 100 and 1000 ppm of chlorides were carried out both on the section and on the lateral surface of the specimens.

The results showed that AISI 316 stainless steel is, among the tested ones, the most corrosion resistant type, on the other hand, the AISI 430F was the stainless steel which showed the lowest corrosion resistance. Analysing the results obtained for each individual material, it has been observed that, generally, the ground finishing provides greater pitting potentials compared to the same test conducted on specimens with drawn finishing.

POTENTIOSTATIC TESTS – They were performed by immersing the specimens, polarized at 0.1 V vs Ag/AgCl, in a solution with increasing chloride content. The objective was to determine the critical chloride content before the corrosive process starts. After a week of immersion, chlorides in solution were added in succession every 80 hours in the following dosages: 10 – 30 – 100 – 300 – 1000 – 3000 – 10000 ppm. Throughout the immersion period, a visual observation of the possible onset of corrosion phenomena and an experimental measurement of the current circulating on each sample was performed. The critical chloride content and the trigger time of corrosion was determined by visual observation of a corrosion attack on the sample, confirmed by the measurement of a circulating current greater than 1 mA.

Similarly to what was previously reported for the potentiodynamic tests, also in this case the results obtained are generally consistent with the P.R.E.N. index, classifying the AISI 316 type, among the tested stainless steels, as the most resistant and the AISI 430F as the least resistant to corrosion. Furthermore, it has been observed once again that, for the same analyzed material, the ground finish generally had a better behavior than the drawn finish, comparing the critical value of chlorides and the time needed to have corrosion measured for the two different delivery conditions. This trend is found on all tested stainless steels, with the exception of AISI 303, where the parameters characterizing this test are greater for the drawn finish.

We would like to thank Rodacciai SpA for the material supplied, and RTM Breda and Politecnico di Milano for the tests carried out. We would also like to thank the research group PoliLaPP (Laboratorio di corrosione dei materiali “P. Pedeferri”) of Politecnico di Milano, coordinated by Prof. Marco Ormellese, within which Eng. Giuseppe Diana participated in the investigation as a graduate.

 

Edit of the article published in Inossidabile, periodico del Centro Inox, l’associazione italiana per lo sviluppo degli acciai inossidabili (Inossidabile n° 219 – marzo 2020)