R. Fratesi

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.

Contemporary use of Ni and Bi in hot-dip galvanizing

Abstract Five series of commercially hot-rolled mild steels, with different silicon and phosphorus content, were tested in four industrial hot-dip galvanizing plants with different bath compositions: two innovative alloys containing low percentages of Ni and Bi (Ni≈0.04% and Bi≈0.1%), one alloy containing Ni (≈0.05%) and a traditional Zn–Pb (Pb≈1.1%) alloy. The coatings obtained were studied by chemical analysis, thickness measurements and by metallographic technique observation. The Zn–Ni–Bi alloy gave very interesting results with all the steels tested. Indeed, the steel reactivity and the coating thickness in the two innovative baths were always lower than those obtained by the traditional Zn–Pb bath and similar to those obtained with the bath containing Ni (Technigalva). These results have been attributed to a synergistic effect of Ni and Bi on the reactivity control of the reactive steels; in fact, even a low Ni content gives the same or even better results than the classical Zn–Ni bath, except for high phosphorus steels. Furthermore, Bi is not detrimental to human health and the environment and can be considered a very good alternative to Pb to improve bath fluidity.

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.

Soil Zinc contamination from corrosion of galvanized structures

The present work relates to galvanized structures with several years of time life subjected to atmospheric corrosion, like galvanized high tension steel pylons. The mass and fate of zinc released is evaluated both via empirical and experimental procedures. The corrosion rate determination requested atmospheric condition characterization, especially for SO 2 concentration and experimental activities focused on soil sampling around pylons. The soil zinc content, total and exchangeable, is determinates by different analytical procedures. The zinc diffusion in environment and the zinc extension under the top soil is evaluated using 1-dimensional mathematical model for miscible species in porous soil.

The Influence of Hydrophobized Concrete on the Corrosion of Rebars

The influence of silane-based hydrophobic products - used as concrete chemical admixtures - on the corrosion of steel rebars was studied. Reinforced concrete specimens with and without a silane admixture were exposed to seawater or to aqueous solutions of de-icing salts containing chlorides. Sound and uncracked or deliberately pre-cracked concrete specimens were manufactured and cured before the exposure to aggressive environments. In the pre-cracked specimens the concrete crack tip was in contact with the steel reinforcement. The results - in terms of corrosion observed on the steel reinforcement - were compared with those obtained on the corresponding uncracked specimens. In uncracked specimens any corrosion process was completely blocked independently of the water to cement ratio and concrete cover provided that hydrophobized concrete was used. This effect was due to lack of water penetration, and then of the chloride ingress, through the pores of the hydrophobized cement matrix. In uncracked specimens without the silane admixture, there was corrosion risk when high water to cement ratio and/or thin concrete cover were adopted. On the other hand, corrosion of steel rebars was surprisingly more severe in cracked specimens manufactured by hydrophobized concrete rather than in the corresponding reference concrete specimens without the hydrophobic admixture. These results can be interpreted by admitting that oxygen diffusion - which is needed to feed the corrosion process - can occur directly as a gaseous phase through the open concrete voids in hydrophobized concrete, whereas in concrete without silane oxygen can diffuse much more slowly only through the water filled concrete voids.

Comparative Evaluation of Traditional and Innovative Corrosion Protection Methods in Cracked Reinforced Concrete Exposed to Chloride Environment

The corrosion resistance of cracked concrete specimens reinforced with bare, stainless, or galvanized steel plates are compared with the corrosion behavior of bare steel reinforcement embedded in concrete specimens coated with a flexible polymer-cement based mortar both before and after specimen cracking. The results in terms of corrosion electrochemical potential and short-circuit electric current measured on the different steel reinforcements are also compared with those related to galvanized reinforcement embedded in hydrophobic concrete specimens. Reinforced concrete specimens were manufactured for each protection method considered and cured before exposure to the test environments. Some specimens were previously cracked by applying flexural stress. The specimens were exposed to increasingly aggressive environments: forty days of full immersion in a 3.5% NaCl solution, simulating a marine environment, were followed by five months of wet-dry cycles using a 10% NaCl solution, simulating a bridge deck treated with deicing salts. The results for the full immersion condition show that negligible corrosion rates were detected in all the cracked specimens, except those treated with the flexible polymer-cement mortar before specimen cracking and the hydrophobic concrete specimens. On the other hand, for the cracked specimens exposed to wet-dry cycles, high corrosion rates were measured for both bare and galvanized steel reinforcement. This was in contrast to the constantly good behavior of stainless steel reinforcement and also of the galvanized steel reinforcement embedded in the hydrophobic concrete.