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。     

Forthcoming corrosion events

Abstract

The XX International Materials Research Congress (IMRC) 2011 was held during August in the tropical paradise of Cancun, in the Yucatan peninsula, Mexico. This is a joint event sponsored by the Sociedad Mexicana de Materiales (MRS-Mexico) and the Materials Research Society (MRS-USA).     

Monitoring corrosion rates and localised corrosion in low conductivity water

Monitoring of low corrosion rates and localised corrosion in a media with low conductivity is a challenge. In municipal district heating, quality control may be improved by implementing on-line corrosion monitoring if a suitable technique can be identified to measure both uniform and localised corrosion. Electrochemical techniques (LPR, EIS, crevice corrosion current) as well as direct measurement techniques (high-sensitive electrical resistance, weight loss) have been applied in operating plants. Changes in the corrosion processes are best monitored in non-aggressive, low conductivity media with sensitive electrical resistance technique and crevice corrosion current measurements.     

AC corrosion. Part 2: Parameters influencing corrosion rate

Pipelines and AC power transmission lines frequently share corridors leading to AC interference corrosion problems, as documented by pipeline failures that have occurred in the USA, Canada and Europe, even when cathodic protection is applied. In order to investigate these phenomena, weight loss tests on carbon steel samples were performed in soil-simulating conditions (aerated and de-aerated sulphate solutions) at various AC current densities from 10 to about 900 A/m². Tests on freely corroding samples showed that the corrosion rate increased as AC current density increased; the effect of AC on corrosion rate was also detected at current densities lower than 30 A/m². The results obtained are analyzed and discussed together with polarization test results, which were presented in a previous paper by Goidanich et al. (submitted for publication) [1].     

Co-operative corrosion phenomena

The corrosion of aluminium alloy 2024-T3 (AA2024-T3) was studied as a function of immersion time from 2.5 to 120 min in 0.1 M aqueous NaCl solution. At immersion times as short as 5 min, rings of corrosion product of 100 to 200 μm diameter, containing smaller domes of corrosion product, were observed using SEM. The domes of corrosion product had greater chloride concentrations than elsewhere on the surface and represented sites of anodic attack. As the immersion time was increased, significant grain boundary attack was observed within the rings of corrosion product. Analyses of Particle Induced X-ray Emission (PIXE) maps of the corroded surfaces showed a significantly higher number of IM particles around the chloride attack sites than the average particle density for the maps, indicating clustering of IM particles. These results suggest a co-operative corrosion effect as a result of clustering of the IM particles. A mechanism for the generation of the corrosion rings is discussed.     

Industrial corrosion monitoring

The paper describes the experience accumulated on electrochemical methods for corrosion monitoring. Electrical resistance, linear polarization, electrochemical potential and amperometric measurements are described and their reliability is evaluated together with that of the related commercial equipments. The main drawbacks of each method (e.g. sensibility, linear or non linear response, causes of error and correction…) are discussed.     

The role of rusts in corrosion and corrosion protection of iron and steel

The processes of atmospheric corrosion of iron and steel and the properties of corrosion products (rusts) are modeled based on a quantitative evaluation of the chemical reactions pertaining to corrosion to elucidate the conditions with which corrosion-protective rust films form. Based on the model, it is suggested that in the initial stage of corrosion, in the rusts, the pH of the aquatic system is maintained at 9.31 owing to an equilibrium with iron(II) hydroxide and the rate of air-oxidation at this pH is very fast, and that dense, self-repairing rust films form, protecting the underlying iron and steel. However, after corrosion stops, the rust film deteriorates due to the dissolution and shrinkage by aging, and the deteriorated rust film separates the anode and cathode reaction products (Fe²⁺ and OH⁻ ions) to cause crevice corrosion. The air-oxidation of iron(II) in anode channels without the presence of OH⁻ ions results in strongly acidic solutions (pH 1.41), causing acid-corrosion. It is proposed that good catalysts (e.g. copper(II) and phosphate ions) accelerate the air-oxidation at low pH, delaying the crevice- and acid-corrosion stages. Further, it is argued that iron compounds with negative charges due to the non-stoichiometric proportions of the lattice oxide ions and metal ions (solid oxoanions of iron) exhibit stable cation-selective permeability even with a drop in pH. Rust films including such compounds would stop the passage of aggressive anions and act to protect iron and steel.     

A new perspective on measuring the corrosion rate of localized corrosion

Many corrosion phenomena are nonuniform, which means that anodic and cathodic locations are spatially separated. An example is macrocell corrosion of steel in concrete. Under these conditions, determining the corrosion rate from polarization resistance measurements and using the Stern–Geary equation is fundamentally not possible. We present a novel theoretical approach for the interpretation of galvanostatic pulse measurements, to make them applicable as a method for corrosion rate measurements in situations of localized corrosion. Experiments show that it is important to consider that (a) only a fraction of the applied current flows through the anode of the macrocell, and (b) this current is not constant over time. We propose an approach to quantify and consider these two effects, based on information generally accessible in condition assessment of concrete structures. Our results show that galvanostatic pulse measurements are a robust method to determine the corrosion current. With the traditional empirical approach, the measurement error was generally below factor 3, and occasionally up to factor 10. With the novel approach, this error could be reduced to a factor of maximum 2 in all cases.     

Polyamine compound as a volatile corrosion inhibitor for atmospheric corrosion of mild steel*

Bis‐piperidiniummethyl‐urea (BPMU) was developed as a volatile corrosion inhibitor (VCI) for mild steel. Its vapor corrosion inhibition property was evaluated by volatile inhibiting sieve test (VIS). Electrochemical measurements were conducted in simulated atmospheric corrosion water. Electrochemical impedance spectroscopy of a volatile corrosion inhibitor monitor cell (VCIM) was applied to study the effect of BPMU on the corrosion inhibition of mild steel under a thin electrolyte layer. The results show that BPMU has good protection effect for steel. It suppressed the anodic reaction of the steel electrode in a manner of promoted passivation. Fourier transform infrared (FT‐IR) spectroscopy was used to characterize the adsorption of BPMU on the steel surface.     

On-line corrosion monitoring in geothermal district heating systems. II. Localized corrosion

Corrosion monitoring in district heating systems has traditionally been performed by using off-line methods, such as weight loss. The disadvantage is that the method is very slow, especially in low-corrosive environments, and that it only provides information about the past corrosion (accumulated over period of time). The purpose of the work is to test on-line monitoring methods in geothermal hot water in the district heating system in Reykjavik, Iceland. Geothermal water poses certain problems with regards to corrosion monitoring due to low conductivity, high pH and the presence of sulphide. These conditions make the geothermal environment low corrosive. However, a quality control is needed, especially since the sulphide causes the steel pipes to be more vulnerable towards localized corrosion. The methods used in this study were: linear polarization resistance (LPR), harmonic analysis (HA), electrochemical noise (EN) and zero resistance ammetry (ZRA). The Pitting Factor was measured with EN and showed promising results for measuring-localized corrosion on-line. A crevice corrosion cell also gave good results but had a low lifetime. However, the results of a galvanic corrosion cell (Fe/Cu) proved more difficult to interpret since they were dependent both on the dissolved oxygen and the sulphide level.     

Use of vapour phase corrosion inhibitors in packages for protecting mild steel against corrosion

Some aspects of the passivation of mild steel in the presence of selected vapour phase corrosion inhibitors (VCIs) were considered. In particular their ability to vapourize was evaluated by sublimation tests and their role in the inhibition mechanism of mild steel was investigated by electrochemical methods such as open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements. In the presence of some carboxylates, amines and azoles alone and as mixtures a protective layer can be formed on mild steel in neutral and alkaline solution. It was shown that the passivation mechanism strongly depends on the pH of the solution. In addition the influence of contaminants from industrial alkaline cleaning baths on the protective properties was analyzed.     

Theoretical description of the practical possibility of stress corrosion cracking from crevice corrosion sites

Stress corrosion cracking (SCC) from crevice corrosion sites had been found in an experimental work at polarization potential of + 200 mV_SCE. In that work, an occluded U‐bend specimen of Type 316L (UNS S31603) stainless steel was used. The testing was done in sodium chloride (NaCl) solution. Based on that work, the practical possibility of SCC from the occluded U‐bend specimen was described theoretically. It was shown that it would also be possible for SCC to occur in practice (i.e. at practical corrosion potential), but the crevice needs to be tighter. Meanwhile, it would take a longer time for obvious SCC to emerge. For a practical crevice usually formed by placing a crevice former on a large uniform metal surface, the crevice geometry may have little effect on SCC although the crevice can sustain an acidified solution more easily than pitting. The possibility of SCC should mainly depend on the corrosion system itself, i.e. material and environment.     

Influence of the corrosion products of copper on its atmospheric corrosion kinetics in tropical climate

In the present paper, the identification of the corrosion product phases formed on copper under different atmospheres of Cuban tropical climate is reported. Cuprite (Cu₂O), paratacamite (Cu₂Cl(OH)₃), posnjakite (Cu₄SO₄(OH)₆ · 2H₂O) and brochantite (Cu₄SO₄(OH)₆) were the main phases identified by X-ray diffraction (XRD) analysis and Fourier transform infrared spectroscopy (FTIR).Copper corrosion products are known to have a protective effect against corrosion. However, a different behaviour was obtained under sheltered coastal conditions. This can be due to the corrosion products morphology and degree of crystallisation, rather than their phase composition. A higher time of wetness and the accumulation of pollutants not washed away from the metal surface can also play an important role.     

Localised effects of macrofouling species on electrochemical corrosion of corrosion resistant alloys

Abstract

Interactions between macrofouling and corrosion have been studied on two duplex stainless steels (UNS S31803 and UNS S32760), an austenitic stainless steel (UNS S31354), a nickel base alloy (UNS N06625), and a cobalt base alloy (UNS R30006) in long term tests conducted in natural sea water off the west coast of Scotland. After an 18 month exposure period, the specimens were heavily fouled primarily with barnacles and mussels and all the materials exhibited crevice corrosion at the specimen/resin interface, although this was less extensive on the nickel base alloy. Localised corrosion was observed under the base of live barnacles on UNS S31603 stainless steel. Direct current electrochemical anodic polarisation tests undertaken after the 18 month exposure period yielded unusually high currents in the range of potentials between the free corrosion value and the breakdown potential. However, long term exposure and associated macrofouling had little effect on the overall breakdown of general surface passivity as indicated by values of the breakdown potential. The higher currents were associated with the appearance, after the anodic polarisation, of black sulphide corrosion products at the specimen/resin crevices, around barnacles, and around mussel byssus threads.     

Monitoring corrosion in multiphase pipelines

The corrosion and inhibition of carbon steel in multiphase flow were examined by Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise (ECN) methods. It is found that the corrosion in slug flow is higher than that in full pipe flow. For the inhibitor tests, the experimental results show that the corrosion inhibitor has a better performance at longer exposure time and higher inhibitor concentration. Comparison with full pipe flow, slug flow degrades the inhibitor performance and increases the corrosion rate. The results from this work show that both EIS and ECN are the practical techniques in monitoring of corrosion and inhibition for pipeline steel in multiphase flow.     

Indoor atmospheric corrosion in Cuba. A report about indoor localized corrosion

Indoor weight loss of steel, chloride, sulphur compounds and dust deposition rate were determined in six storehouses having different characteristics. Relative humidity and temperature were determined in three storehouses. A model for indoor corrosion of steel depending on time of exposure and deposition of dust, sulphur compounds and chlorides is proposed. Dust deposition plays an important role indoors. The position of the sample has also a significant influence on corrosion. Indoor corrosion aggressivity in Cuban storehouses ranges in classification IC3 and IC4 according to the new ISO proposal of indoor aggressivity.A report about the presence of localized corrosion indoors (filiform like) using a special designed sample is made.     

Long-term atmospheric corrosion of mild steel

A great deal of information is available on the atmospheric corrosion of mild steel in the short, mid and even long term, but studies of the structure and morphology of corrosion layers are less abundant and generally deal with those formed in just a few years. The present study assesses the structure and morphology of corrosion product layers formed on mild steel after 13 years of exposure in five Spanish atmospheres of different types: rural, urban, industrial and marine (mild and severe). The corrosion layers have been characterised by X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). Long-term corrosion is seen to be more severe in the industrial and marine atmospheres, and less so in the rural and urban atmospheres. In all cases the corrosion rate is seen to decrease with exposure time, stabilising after the first 4–6 years of exposure. The most relevant aspects to be noted are (a) the great compaction of the rust layers formed in the rural and urban atmospheres, (b) the formation of hematite and ferrihydrite phases (not commonly found) in the industrial and marine atmospheres, respectively and (c) identification of the typical morphological structures of lepidocrocite (sandy crystals and flowery plates), goethite (cotton balls structures) and akaganeite (cotton balls structures and cigar-shaped crystals).     

Stress corrosion cracking and selective corrosion of copper-zinc alloys for the drinking water installation

Despite a generally good corrosion resistance to tap and industrial water, many brass taps and fittings have failed in the past by stress corrosion cracking (SCC) and selective corrosion (dezincification or preferred removal of a phase). The experimental investigations of the present study clarify the influence of the ammonia concentration on the two types of corrosion. Notched specimens made of the alloys CuZn39Pb3, CuZn40Pb2, CuZn37, CuZn36Pb2As and CuZn21Si3P are polarized anodically in pure tap water and tap water with realistic ammonia concentrations (15 and 30 ppm) under a simultaneous mechanical loading condition. The influence of stress and of the third alloying elements lead and arsenic are investigated and evaluated. The experiments show that the ammonia additions significantly increase the risk of dezincification of the α-β-brasses. The arsenic in the CuZn36Pb2As alloy avoids dezincification, but enhances the risk of SCC. The rate of selective corrosion and SCC consistently increases with increase in tensile stress.     

Perturbation method analysis of AC-induced corrosion

A nonlinear model for corrosion of metals subjected to alternating voltage (AV) induced corrosion that includes the elements corresponding to the polarization resistance, the double layer capacitance and the solution resistance is developed. The results show that corrosion of materials under AC conditions is greatly enhanced by the peak potential and by an increase in the ratio of the anodic-to-cathodic Tafel slope. The AC-induced corrosion rate: (i) decreases with the frequency of the applied signal and with an increase in the DC corrosion potential, (ii) is independent of the DC corrosion rate, and (iii) increases with the peak potentials of the AV signal.     

Conditions for stress corrosion cracking to occur from crevice corrosion sites and related electrochemical features

The occurrence of stress corrosion cracking (SCC) from crevice corrosion sites was studied using a kind of occluded U‐bend specimen of Type 316L (UNS S31603) stainless steel. It was done in 0.5 M NaCl solution at room temperature and 50°C. The galvanostatic and potentiostatic polarization methods were adopted. It was found that a tight crevice and high polarization current were the necessary conditions for SCC to occur relative rapidly from crevice corrosion sites. Meanwhile, if there was obvious SCC developing from a crevice corrosion site, the IR drop (or solution resistance) in the crevice path would increase. The increase of IR was attributed to the corrosion product deposition and/or hydrogen bubbles evolved from the cracks. On the other hand, the usually high aspect ratio of stress corrosion cracks, i.e. the inhibited dissolution on the crack walls, might be explained as due to the existence of high IR drops in them.