G. Roventi

Electrochemical study on the corrosion resistance of Cr III-based conversion layers on zinc coatings

Abstract Zinc coatings submitted to a Cr III-based passivation treatment, with and without a sealing treatment, were studied. For comparison, similar coatings with a Cr VI-based passivation treatment were also examined. The corrosion behaviour was studied by polarization resistance (Rp) measurements, anodic polarization and mainly by electrochemical impedance spectroscopy (EIS) in aerated 0.85 M NaCl solution. Salt spray tests and morphological observations on the coating surface were also performed. The results indicate that the Cr III passivated coatings with sealing treatments have a corrosion resistance comparable with that of coatings with a Cr VI passivation treatment without a sealant; therefore, they could be a less polluting alternative to the traditional chromated coatings.

Zinc-cobalt alloy electrodeposition from chloride baths

Electrodeposition of Zn–Co alloys on iron substrate from chloride baths under galvanostatic and potentiostatic conditions were carried out. Current density, temperature and cobalt percentage in the bath were found to strongly influence the composition of the deposits and their morphology. Changes in potentials, current efficiency and partial current densities were studied. The results show that the shift in potential and in the cobalt percentage of the deposits, for a particular current density during galvanostatic electrodeposition, does not always correspond to the transition from normal to anomalous codeposition. This shift is attributed to zinc ion discharge, which passes from underpotential to thermodynamic conditions. In the range of potentials for the underpotential deposition of zinc, the electrodeposition of zinc–cobalt alloys is discussed, emphasizing the influence of the electrode potential on the composition and microstructure of the deposits.

Corrosion behaviour of electrogalvanized steel in sodium chloride and ammonium sulphate solutions; a study by E.I.S.

Zinc and zinc-nickel (13% Ni) electrodeposits were passivated by dipping in chromate baths and characterized by scanning electron microscopy. The corrosion behaviour was studied using a.c. electrochemical techniques; electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit and under galvanostatic control during the 24 h immersion time. In sodium chloride solution the zinc-nickel electrodeposits show a better corrosion resistance compared to the pure zinc coatings. During the immersion time, a surface nickel enrichment was observed which, together with the zinc corrosion products, acts as a barrier layer reducing the total corrosion rate. In the same solution the passivation treatment improves the corrosion resistance of the electrodeposits; nevertheless, on zinc substrates, the protection exerted by the chromate film is not, always effective during the immersion time. On the contrary the chromate coating on zinc-nickel substrates induces a remarkable and durable improvement of the corrosion resistance reducing the zinc dissolution almost completely. In the ammonium sulphate solution, the corrosion mechanism is significantly influenced by hydrogen reduction on the zinc-nickel surfaces, and by the production of a local surface acidity which is aggressive for the chromate coatings.

Field and laboratory corrosion tests on zinc-nickel alloy coatings

Abstract Among the zinc alloy coatings, zinc-nickel is of particular interest because it seems to maintain very good corrosion resistance even when it is exposed to heating, as happens to components in car engine compartments. Chromated zinc and chromated zinc-nickel alloy coatings were placed on the underside and in the engine compartment of different cars subjected to field testing. The results were compared with those obtained in laboratory tests on the same materials, before and after heat treatment at 120°C for 1 h. The influence of heat treatment on the superficial morphology and corrosion resistance of the coatings was evaluated using scanning electron microscopy, salt spray tests and electrochemical tests. The results obtained in this work show that the heat treatment does not produce any remarkable modification of the behaviour of chromated zinc-nickel alloys whose corrosion protective properties remain almost unchanged, in contrast with the behaviour of chromated zinc.

Localised corrosion and cathodic protection of 17 4PH propeller shafts

Abstract Some 17 4PH propeller shafts showed severe localised corrosion after a long period of time in Ancona harbour. The shaft is concentrically mounted in an AISI 304 stern tube through two rubber bearings. Both 17 4PH and AISI 304 stainless steels were characterised by anodic polarisation in natural sea water, in order to obtain the necessary corrosion parameters for designing a suitable cathodic protection of the propeller system. The cathodic protection effectiveness was tested on a portion (about 1 m long) of this system. Two cathodic protection methods were tested: an impressed current cathodic protection at −0·3 V(SCE) and a galvanic cathodic protection. The tests were performed both in stagnant and in moving sea water and the results showed that these cathodic protection solutions were able to protect the propeller system; in particular some guidelines are suggested for both old and new ships.

Effect of the Type of Surface Treatment and Cement on the Chloride Induced Corrosion of Galvanized Reinforcements

The effect of a new passivation treatment, obtained by immersion of the galvanized reinforcements in a trivalent chromium salts based solution, on the chlorides induced corrosion has been investigated. To investigate also the effect of cement alkalinity on corrosion behaviour of reinforcements, concretes manufactured with three different European cements were compared. The obtained results show that the alternative treatment based on hexavalent chromium-free baths forms effective protection layers on the galvanized rebar surfaces. The higher corrosion rates of zinc coating in concrete manufactured with Portland cement compared to those recorded for bars in concrete manufactured with pozzolanic cement depends strongly on the higher chloride content at the steel concrete interface.