The results of an electrochemical study performed on duplex, superduplex, austenitic and superaustenitic stainless steel in halide containing sulfuric media are presented and compared. In particular, the specific role of chloride and fluoride ions on both localized and uniform corrosion was investigated. From these results, the use of the duplex stainless steels family in the chemical industry, flue gas desulphurization process and hydrometallurgy is discussed.
During the last 20 years, the effect of chloride concentration and temperature on the corrosion resistance of stainless steels in chloride containing sulfuric acid solutions has been widely investigated. More recent studies provided additional data, particularly for modern duplex and superduplex grades. Thus, for numerous stainless steel grades, potential ? pH diagrams have been plotted for two concentrations of chloride (3% and 6%), two temperatures (140°F/60°C and 176°F/80°C) and for 0.5<pH<5.
The aim of this paper is to evaluate the influence of fluoride additions in chloride containing sulfuric acid media on the corrosion of stainless steel grades at 140°F/60°C. In order to complete the available corrosion data concerning these materials an extensive electrochemical study has been carried out.
MATERIALS AND CHEMICAL ANALYSIS
The stainless steels considered in this study are austenitic, superaustenitic, duplex and superduplex grades. Those materials are:
- UNS S31603 / 316L and UNS S31726 / 317LMN for austenitic grades,
- UNS N08904, UNS N08926, UNS S31266 or respectively Uranus® B6, B26 and B66 for super austenitic grades,
- UNS S32205 / Uranus® 45N+ and UNS S31803 / Uranus® 45N for duplex grades,
- UNS S32520 / Uranus® 52N+ for the superduplex grade (may be dual certified as UNS S32550).
All stainless steels tested in this work are hot rolled materials. Table 1 gives the typical chemical analysis of these materials.
Specimens were immersed in aerated sulfuric acid solution containing 30g/l NaCl and 0.1% Fe2(SO4)3 at 140°F/60°C. The duration of the test is 15 days and the pH is adjusted to 2 by H2SO4 additions. The first tests have been carried out with 25mm x 50mm x thickness coupons immersed in a fluoride free solution or in a 0.1% fluoride containing solution. The uniform corrosion rate for each material is calculated from the weight loss. The second tests have been performed using samples fitted with a multi crevice assembly (MCA). In this case, the corrosion rate and percentage of attacked crevices have been evaluated for each coupon immersed in both solutions (with or without fluoride). The percentage of attacked crevices is the number of corroded crevice sites divided by the total number of potential crevices sites (which is determined by the multi crevice washer geometry) multiplied by 100.
An electrochemical cell with a capacity of 150ml was used. Only 2 cm2 of one side of the coupon was exposed to the electrolyte. For tests in the fluoride containing solution, materials that do not react with fluoride were used: polypropylene for the cell and Teflon for the reference electrode. All samples were mechanically polished with silicon carbide (SiC) papers to a 600 grit, followed by cleaning with ethanol. The electrochemical tests began 24 hours after sample preparation in order for the passive film to form naturally as a result of electrochemical reactions with the atmosphere. A platinum sheet was used as a counter electrode. The reference electrode was a saturated calomel electrode (SCE) for fluoride free containing solutions and an Ag/AgCl electrode for f