On the Nature of Surface Films Formed on Iron in Aggressive and Inhibiting Polyphosphate Solutions

Abstract

The corrosion behavior of pure iron in diluted (inhibiting) and moderately concentrated (aggressive) polyphosphate solutions has been re-examined in the light of the results of surface analyses of iron samples by means of x-ray photoeleetron spectroscopy. The passive film responsible for the anodic control of the corrosion of iron in diluted (NaPO3)x solutions appears much like an air-formed oxide film with, in addition, some inclusions of iron phosphate. The photoelectron spectra of iron samples exposed to (NaPO3)x/CaC12 solutions suggest the reversion of polyphospharos to orthophosphates as being an essential step in the formation of the protective film. A possible mechanism of the growth of such a film, composed of a mixture of calcium and iron orthophosphates, is proposed.Among various inorganic ions acting as corrosion inhibitors of iron or steel, polyphosphates (metaphosphates) possess a rather unique property of being effective when added in small amounts to even concentrated chloride solutions (1-3). However, the effectiveness of polyphosphates depends to a large extent on the simultaneous presence in the solution of some divalent cations, principally Ca 2+ and Zn 2+ (1, 2, 4), as well as on the concentration of dissolved oxygen (2, 5) and on the rate of stirring or flow (1,5,6). It is to be mentioned that high concentrations of polyphosphate increase, in a significant manner, the corrosion rate of iron in water, which is in contrast with the behavior of most of other inhibiting ions.There has been a large variety of interpretations tending to explain the mechanism by which polyphosphates exert their inhibiting action. These include: (i) adsorption of hexametaphosphate or of a complex thereof on the metal or metal oxide surface (6); (ii) formation of a film of adsorbed oxygen favored by the interaction between polyphosphate and ferrous oxide initially present on the iron surface (2, 7) (iii) deposition on the local cathodic areas of the metal of a protective glassy phosphate coating, proceeding through the migration of a calcium phosphate complex present in the solution either as a cation or as a positively charged colloidal particle (8); (iv) formation of a calcium carbonate film induced by the presence of polyphosphate ions in the solution (9). The latter hypothesis has, however, been rejected by Uhlig et al. (2) who argued that such a film would be dissolved by polyphosphate. As a matter of fact, small amounts of polyphosphate added to water have been extensively used for the prevention against and elimination of calcium Carbonate scale (10).Enhanced efficiency of polyphosphate in reducing the corrosion rate (especially in chloride solutions), obtained by the conjoint use of the divalent Ca2+ or Zn2+ ions, has been connected with the formation on the cathodic areas of a diffusion barrier layer impeding access of oxygen to the iron surface (2). According to Uhlig et al.(2), such film, supposed to contain both calcium and phosphorus (8, 11), would exert a mixed (anodic and cathodic) control of the corr...

The Unexpected Role of Carbonate Impurities in Polyphosphate Corrosion Inhibition

“…The presence of orthophosphates can be ascribed to the hydrolysis of P-O-P bonds in polyphosphates, which is known to occur under alkaline conditions 21 . This reversion to orthophosphate species at the cathode has been proposed 11 , but not confirmed, before, since local pH values are known to exceed 10 when corrosion at the cut edge is uninhibited 22 , 23 . Previously however, films attributed to polyphosphate species (morphologically distinct from the precipitates formed by orthophosphate inhibitors) have been detected in ex-situ tests after lengthy exposure times 15 , 19 , 24 .…”

Electrochemical and surface analytical studies of synergistic effect of phosphonate, Zn2+ and ascorbate in corrosion control of carbon steel

“…In case of the binary formulation, the P 2p peak is observed at 133.0 eV, while in case of the ternary formulation, it is observed at 133.1 eV. In the literature 52, 53, it was reported that the P 2p peak could be observed in the range of 132.9–133.8 eV, for iron or steels immersed in the solutions containing phosphonates, orthophosphates and polyphosphates. Nakayama 54 obtained a P 2p peak at 133.0 eV and attributed it to the presence of phosphonate compounds.…”

Surface Analysis of Carbon Steel Protected from Corrosion by a New Ternary Inhibitor Formulation Containing Phosphonated Glycine, Zn<sup>2+</sup> and Citrate

“…Two P 2p peaks are observed one each at 132.4 eV and 133.2 eV. In the literature [47], it was reported that the P 2p peak could be observed in the range of 132.9 to 133.8 eV, for iron or steels immersed in the solutions containing phosphonates, orthophosphates and polyphosphates. Nakayama obtained a P 2p peak at 133.0 eV and attributed it to the presence of phosphonate compounds [48].…”