M.J. Robinson

The corrosion resistance of electrodeposited zinc-nickel alloy coatings

Abstract The corrosion behaviour of electrodeposited zinc-nickel (Zn-Ni) alloy coatings has been studied in aqueous chloride environments. The corrosion rates of detached zinc alloys containing up to 25% Ni by weight were determined using linear polarisation techniques. The corrosion rate of Zn-Ni alloys was found to decrease with increasing Ni content over the compositional range studied. Galvanic corrosion measurements have indicated, however, that Zn-Ni alloy coatings become less sacrificial toward steel as the Ni content is increased. These results are used to interpret the corrosion behaviour of electroplated steel in a neutral salt fog environment.

Corrosion product force measurements in the study of exfoliation and stress corrosion cracking in high strength aluminium alloys

Abstract Exfoliation corrosion was assessed in three high strength aluminium alloys by measuring the force generated by voluminous corrosion products and their stress corrosion behaviour was studied in tests on double cantilever specimens. There was an inverse linear relationship between the corrosion product forces and the K ISCC values for stress corrosion cracking in these materials, providing further evidence for a common corrosion mechanism. Exfoliation and SCC were both dependent on the rate of intergranular corrosion, which is controlled in these alloys by the grain boundary precipitates and the resistance to both forms of corrosion was improved in alloys with an over-aged heat treatment condition.

Hydrogen embrittlement of high strength steel electroplated with zinc-cobalt alloys

Slow strain rate tests were performed on quenched and tempered AISI 4340 steel to measure the extent of hydrogen embrittlement caused by electroplating with zinc-cobalt alloys. The effects of bath composition and pH were studied and compared with results for electrodeposited cadmium and zinc-10%nickel. It was found that Zinc-1%cobalt alloy coatings caused serious hydrogen embrittlement (EI 0.63); almost as severe as that of cadmium (EI 0.78). Baking cadmium plated steel for 24 hours at 200°C gave full recovery of mechanical properties but specimens plated with zinc-1%cobalt and then baked still failed in 89% of the time of unplated controls. It was shown that hydrogen uptake and embrittlement could be controlled by depositing thin layers of cobalt or nickel at the steel/coating interface. For example, the least embrittlement was caused by zinc- 10%nickel (EI 0.037) due to a nickel rich layer with very low hydrogen diffusion coefficient that formed during the initial stages of electroplating. Similarly, a 0.5 micron nickel layer was effective in lowering the embrittlement caused by zinc-1%cobalt to that of zinc-10%nickel. Furthermore, a 0.5 micron cobalt layer deposited before a zinc-1%cobalt coating gave virtually 100% recovery of mechanical properties after baking.

Mathematical modelling of exfoliation corrosion in high strength aluminium alloys

Abstract A mathematical model was developed for the formation of surface blisters during the exfoliation corrosion of wrought high strength aluminium alloys. The internal pressure of the blisters was calculated together with the dimensions that define the extent and the severity of the degradation. The effects of the elongated grain shape and of the heat treatment condition on exfoliation corrosion were investigated theoretically and found to compare favourably with experimental observations.

Corrosion rate measurements of electrodeposited zinc-nickel alloy coatings

Abstract The corrosion rates of detached electrodeposited zinc-nickel alloys containing up to 30% nickel by weight were determined under total immersion conditions in NaCl solutions and on exposure to neutral salt fog. Weight loss measurements and linear polarisation resistance (LPR) techniques were employed to determine corrosion rates under both experimental conditions. The corrosion rates of zinc nickel alloys were found to decrease with increasing alloy nickel content under total immersion conditions in NaCl solutions, indicating that the barrier corrosion resistance of zinc alloys increases with increasing nickel content. This effect was attributed to both a reduction in the rate of zinc dissolution due to nickel enrichment at the metal surface and an increase in the stability of the corrosion product layer. Pure zinc was found to exhibit a higher corrosion rate in the salt fog environment than in quiescent NaCl solutions and this was thought to result from the higher level of available oxygen in the salt fog environment. In contrast, the corrosion rate of a zinc-14 wt% nickel alloy was lower in the salt fog environment than under quiescent conditions due it is suggested to a more rapid ennoblement of the surface and the formation of a protective corrosion product layer.

The influence of lattice hydrogen content on the hydrogen-assisted cracking of high strength steel

Abstract The susceptibility of BS4360 grade 50D carbon-manganese steel to hydrogen-assisted cracking was measured in a range of aqueous environments including artificial seawater at applied cathodic potentials. The diffusible lattice hydrogen content of the steel was determined electrochemically and related to the threshold stress intensity, K w , to cause cracking. Permeation studies were used both to assess the tendency of each environment to promote hydrogen absorption and to study the effects of calcareous deposit formation. A calcium/magnesium rich deposit was found on the crack walls but was not detected within 2 mm of the crack tip. Hydrogen-assisted cracking of 50D steel did not appear to be influenced by the presence of such deposits.

Hydrogen absorption resulting from the simulated pitting corrosion of carbon-manganese steels

Abstract Hydrogen permeation measurements were performed on membranes of BS4360 Grade 50D C-Mn steel in the quenched and tempered condition. The rates of hydrogen absorption resulting from exposure to FeCl 2 solutions in a simulated corrosion pit were measured and found to be lower than those occurring in artificial sea water at applied potentials in the range commonly used for cathodic protection. A progressive decrease in the hydrogen permeation flux was recorded during simulated pitting and was attributed to the formation of a partially protective film of magnetite on the steel surface. At cathodic applied potentials iron plating was observed on the membranes. It is suggested that a similar process occurs in the cathodic protection of steel containing real corrosion pits and leads to a lowering of the Fe 2+ ion concentration within the pits and a decrease in the aggressiveness of the local environment.

Ion exchange resin fibres for potential measurement in crevice corrosion studies

Abstract Fibres of cation and anion exchange resins, regenerated in the Na + and Cl − forms respectively, have been used to monitor corrosion potentials during crevice corrosion of stainless steels. The resin fibre probe has a relatively high electrical resistance but has the significant advantage that it can be produced flush with the crevice wall, thus avoiding the entrainment of additional electrolyte.