Implications of the kosmotrope/chaotrope nature of the anions on the breakdown of passivity of iron by halides

Abstract

The comparative aggressiveness of NaF and NaCl solutions on the breakdown of passivity of iron has been investigated once the passivating borate buffer solution has been removed. The results highlight the inhibitive action of borate anions towards the less aggressive kosmotrope fluorides. Without borates, the kinetics of attack by fluorides is slower as the prepassivation is longer, and it is considerably retarded when compared to the case with the chaotrope chlorides. The high positive viscosity B coefficient value of borate anion argues in favor of its use as an inhibitor or as an efficient electrolyte for anodization baths. 2. Material and methods The chemical compounds: sodium fluoride (NaF), sodium chloride (NaCl), sodium bromide (NaBr), sodium tetraborate decahydrate (Na 2 B 4 O 7 ,10H 2 O), boric acid (H 3 BO 3), were analytical grade products manufactured by Sigma-Aldrich (France). Deionized water was taken as a solvent for the preparation of the electrolyte solutions. The pH of the solutions was measured with an electronic pH-meter (Eutech Intruments). NaCl solution at 0.1 M concentration with a pH 10 was prepared by adding some droplets of a sodium hydroxide (NaOH) solution at 0.1 M concentration. Small iron plates were cut from a commercial Fe foil (99.99%, Goodfellow, UK) with a thickness of 0.25 mm to obtain rectangular shape (1.5 cm  0.5 cm). Before use, the plates were mechanically polished with 1200 grit SiC. Then they were washed in acetone and rinsed with deionized water.

Room temperature corrosion behaviour of plastically deformed AISI 304 stainless steel by friction stir welding in neutral and acidified chloride solutions

“…The difference in the electrochemical performance between the two different electrolytes could be associated with the kosmotropic or chaotropic nature of the ions in the electrolytes [39]. The sulphate (SO 4 2− ) ions have higher surface charge density and exhibit stronger interaction with water molecules than chloride ions.…”

“…The difference in the electrochemical performance between the two different electrolytes could be associated with the kosmotropic or chaotropic nature of the ions in the electrolytes [39]. The sulphate (SO 4 2−…”

The comparative breakdown of passivity of tin by fluorides and chlorides interpreted through the ‘law of matching affinities’ concept

“…5) suggests that free fluoride anions are not engaged in a dominant pitting process but rather in a slower thinning process of the oxide film. Previous studies on passivated metals such as iron and nickel support this mechanism, where the slow and continuous increase of the current density was ascribed to the dissolution of the film through the formation of soluble complexes or salts with metallic cations present in the passive film [54][55][56][57][58][59][60][61]. The enhanced transfer of metallic cations from the oxide to the solution results from a homogeneous attack of the passive layer leading to a decrease of oxide thickness with time.…”

Can the Point Defect Model Explain the Influence of Temperature and Anion Size on Pitting of Stainless Steels

“…along with the radii of the dehydrated anions as reported by Conway 12) . Since an ion's dehydration energy is simply the negative its hydration energy, it is clear that the smaller halides are harder to dehydrate, thus this term favours pitting by larger ions; the large dehydration energy of fluoride ions has been proposed to be the reason why these halide causes general corrosion rather than localised corrosion of a number of titanium and aluminium alloys 13) . Furthermore, as DS Dehyd is positive increasing the temperature will make pitting more favourable but there should be no change in the order of aggressiveness of the ions.…”

“…To explain the influence of temperature on the order of the relative aggressiveness of the various halides all three energy terms need to be considered, which from Equations (3), (8) and (11) can be written as: (12) Neglecting the final entropy term (in practice r < 1.5 r o so this term always less than 4 kJ mol -1 ) and differentiating with respect to r leads to: (13) where a" and b" are again constants. The optimum radi- (14) As long as a ̋ is larger than b T Equation (14) will yield a real number and thus an optimum radius for pitting will exist.…”