Forthcoming corrosion events

Abstract

Pitting potentials have been measured and some gravimetric testing has been carried out on a series of experimental austenitic stainless steels with varying chromium, molybdenum and nitrogen contents. All three of these elements were found to contribute to the resistance to the initiation of pitting corrosion, and synergistic effects have been noted. The effect of nitrogen is especially potent in a steel with 22% chromium and 3% molybdenum.     

Initiation events for pitting corrosion of stainless steel?

Events which could conceivably trigger off pitting corrosion of 18Cr-8Ni stainless steel in chloride-containing solution have been observed upon potentiostatic polarization of microelectrodes (10 μm diameter) of the steel. These events were undetectable on larger electrodes because they could not be distinguished above the background current. The events had amplitude 30–80 pA, frequency up to about 0.05 s⁻¹, and lifetime around 50 s. The average charge carried by an event was 100 pC, rather less than the charge required to dissolve one monolayer of the steel over the entire micro-electrode surface (600 pC). Most of the events were characterised by a sharp rise and slow fall of the current, in contrast to the current transients (slow rise, sharp fall) associated with the nucleation and temporary propagation of micropits on larger electrodes. It is inferred that the transients were associated with a microstructural feature in the microwire used to fabricate the electrodes. The results are rationalized with a model of very slow dissolution of one component of a binary alloy, and rapid dissolution of the other, with a different rate for different types of surface site.     

Determination of instantaneous corrosion rates and runoff rates of copper from naturally patinated copper during continuous rain events

Instantaneous corrosion rates of naturally patinated copper of varying age (16 months, 138 and 145 years) have been determined during continuous rain events in the laboratory with electrochemical impedance spectroscopy using a two-electrode cell. The two-electrode cell was found to yield the same information in bulk rainwater as a conventional three-electrode cell.Relatively constant corrosion rates, between 0.2 and 0.6 μm/y, were determined for samples having a two-layer structure with an inner brownish layer of cuprous oxide and an outer greenish layer of basic copper salts (138, 145 years). Samples with cuprous oxide as the dominating phase of the patina (16 months) showed higher and somewhat increasing corrosion rates during a rain event (from 0.6 to 1.2 μm/y). During a continuous rain event, corrosion rates were found to be approximately 10 (brownish patina) and 25 times (greenish patina) lower than corresponding instantaneous runoff rates. The first flush phenomena of the runoff process, with an increased concentration during first flush and a relatively constant concentration during steady state, was indirectly seen as an increase in solution conductivity during the first rain volume followed by relative constant value. The contribution of the concentration in the first flush to the total annual runoff rate was significant for panels having a greenish layer (138, 145 years) whereas it was negligible for panels having a brownish layer (16 months).     

钢芯铝绞导线大气腐蚀产物层的结构及腐蚀机理

在模拟大气腐蚀环境中,采用干/湿NaHSO3+NaCl水溶液盐雾试验研究钢芯铝绞(ACSR)导线腐蚀产物的相组成及腐蚀层结构,讨论其腐蚀机理。结果表明:ACSR导线中单股铝线或镀锌钢芯线的腐蚀主要表现为点蚀,腐蚀产物组成复杂,主要为锌和铝的氢氧化物、硫酸盐与氯化物的复式盐;在腐蚀初期,内外层铝股线及钢芯线表面镀锌层开始形成点蚀坑,逐步形成连续的腐蚀层;由于镀锌层和内层铝股线之间构成原电池,因为牺牲阳极效应,镀锌层腐蚀速率最大;而内层铝股线受到保护,腐蚀速率最小,外层铝股线腐蚀速率居中。     

Enhanced predictive corrosion modeling with implicit corrosion products

An advanced mathematical approach to describe the influence of corrosion products on the corrosion rate is presented here. The related model can be used as input equation for numerical predictive corrosion simulations or simply as an empirical model, to extrapolate experimental data of corrosion tests to longer times and to interpret the physical parameters behind. This semiempirical model assumes that a constant share of the dissolved metal precipitates on the surface and hinders the diffusion processes. Hence, the effective corrosion rate decreases exponentially with increasing dissolution. The explicit corrosion progress over time is derived by time integration on a newly developed, time dependent corrosion rate equation. The derived expression can be effortlessly implemented in existing for example finite element method, which is demonstrated for the uniform corrosion of a zinc surface. Furthermore, this approach is qualitatively compared with other empirical models for corrosion products and the validity is demonstrated by fitting of experimental data. A very good agreement between experiment and theory can be achieved for various materials and environments considering no change of the driving corrosion mechanism.     

Marine corrosion

Abstract

The accelerating effect of nitrates on the dissolution of tin sometimes observed in tinplate cans of acid products has been investigated by measurements of cathodic polarisation and by direct measurements of corrosion rate in oxygen-free solutions of sulphates and organic acids. The rate of reduction of nitrate, and hence the corrosion rate, increased as the corrosion potential of tin became more positive, possibly as a result of potential dependence of nitrate adsorption on tin. This finding may explain the known influences of pH, of dissolved tin and of the nature of the organic acids present, on the stimulation of corrosion produced by nitrate. The addition of anionic surface active agents created a negative shift of potential and inhibited corrosion but cationic agents caused a positive shift and stimulated corrosion. Sodium lauryl sulphate had a strong inhibitive effect at a concentration of 5 ppm and at 30 ppm almost completely suppressed the effect of nitrate.