Relationship between chemical structure of imidazoline inhibitors and their effectiveness against hydrogen sulphide corrosion of steels

Abstract

Seven homologous imidazoline type inhibitors were synthesised, all being derivatives of cyclopentyl- and cyclohexylnaphthenic acids. The protective effectiveness of these compounds was investigated by applying the potential sweep method to the carbon steel St3S and austenitic steel 1H18N9T in a 2% solution of sodium chloride in contact with a hydrocarbon phase and saturated with hydrogen sulphide gas. It was found that the inhibitors investigated and their mixtures present high protective effectiveness at a concentration of 25 ppm, reaching values of 99% for carbon steel and 80% for austenitic steel at 40°C. This protective effectiveness was improved by increasing the length of the substituent chain. Consequently, the least effective compounds were those with the shortest chain substituted at the imidazoline ring. Inhibitors under study were chemisorbed on the electrode surface of the steel and this chemisorption process obeyed the Frumkin adsorption isotherm. Values of the Gibbs free energy of adsorption of inhibitors were determined, and also the equilibrium constants of adsorption and constants of attraction.     

Stress corrosion cracking of AZ61 magnesium alloy friction stir weldments in ASTM D1384 solution

Abstract

Stress corrosion cracking (SCC) behaviour of a wrought magnesium alloy (AZ61) friction stir weldment was assessed in ASTM D1384 test solution at two strain rates. The analyses have shown that both the parent and the weldment are susceptible to SCC at a nominal strain rate of 10^–6 s^–1. Fractographic evidence clearly reveals the susceptibility, especially of the fine grained region of the friction stir weld nugget to SCC. The susceptibility to cracking has been observed to increase with decreasing strain rate and under this condition (10^–7 s^–1), the behaviour of the parent and the friction stir weldment were nearly the same.     

Micromechanical testing of stress corrosion cracking

Abstract

It is known that grain boundaries with differing chemistry, misorientation and structure have varying susceptibility to stress-corrosion cracking (SCC). However, up till now it has not been possible to obtain mechanical property data on individual grain boundaries as they fail under SCC. A novel method of using focused-ion beam machining to manufacture test specimens containing single grain boundaries, combined with loading in a nano-indenter, allows threshold stress levels and crack growth rates in 304 stainless steel to be directly measured. This technique opens up a new field in being able to validate atomistic scale and dislocation models of intergranular SCC. Combining this information with recent advances in microcharacterisastion, modelling and thermomechanical treatment engineering promises to provide a more complete understanding of inter-granular SCC failure and a better approach to reducing SCC susceptibility and predicting component lifetimes.     

Chemical and structural properties of ancient metallic artefacts: multitechnique approach to study of early bronzes

Abstract

The results are presented of a characterisation study of the microstructure and microchemistry of archaeological bronze (Cu–Sn) artefacts from the eighth to the sixth century bc. Metallographic examination, with optical and electron optical microscopy, has been performed on polished sections of early Iron Age studs and bracelets found in incineration tombs of the Necropolis of Chiavari in Italy. A heterogeneous microstructure of the bronze was observed, exhibiting a wide range of grain sizes, and a predominant α-phase solid solution containing α/δ and α/? eutectoid phases decorated with a high density of inclusions. The composition of grain boundary surfaces was determined, using scanning Auger microscopy (SAM), on specimens fractured in vacuo. The extent of tin segregation at the grain boundaries was 3–5 times greater than that in the grain interiors. Copper rich sulphides occasionally containing the oligoelements (iron and lead) were identified as the predominant type of inclusions formed at the grain interfaces and within the grains. Analysis of the corrosion patina at the surface of the bronze artefacts was conducted by combining X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and SAM. The patina was found to exhibit a multilayered structure and a complex chemical composition forming various crystallographic phases including malachite, cuprite, and copper–tin oxide. Corrosion of the underlying bronze matrix proceeded along the grain boundaries, where the sacrificial corrosion of tin reacting with diffused oxygen and chlorine was identified. The results of this study have been used to clarify the metallurgy and manufacturing processes of the examined finds, and to evaluate the state of their degradation.     

Acceleration of galvanic lead solder corrosion due to phosphate

Although orthophosphate is often effective in reducing lead corrosion, bench-scale tests revealed cases in which even high doses of orthophosphate (1–3 mg/L P) in potable water increased lead and tin release from simulated soldered copper joints. Phosphate increased the galvanic current between tin and copper plumbing materials, especially in water with less than 10 mg/L and when the percentage of the anodic current carried by ion was less than 30%. Tin release was increased more than lead release from 50:50 Pb–Sn solder in these circumstances.