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Galvanic corrosion It is commonly held that it is the electrochemical potential between two surfaces that is the controlling factor for the rate of corrosion. Table 1.2 in chapter one of this series lists the standard oxidation potentials. However, the difference between the potentials of the two metals plus the difference in the e.m.f. due to the concentration of ions is the reversible electrochemical potential, which only applies when there is no current flowing. The degree of corrosion that occurs is based on the potential difference existing when there is a known current flowing. Thus the baser of two connected metals can be extremely corrosion‐resistant, even if the potential difference is quite large, provided at least one of them has good polarisation characteristics. Metals that are particularly damaging to ferrous metals not only have a very low potential, but are also to all practical purposes insoluble in the corrosive environment around the steel. Thus it is that one of the worst is copper.

The increase in automobile corrosion problems stems largely from greater use of salts for ice and snow removal. Chemicals and water trapped in hidden cracks and crevices can continue to cause corrosion for long periods. This article, originally published in the December 1966 issue of ‘Materials in Design Engineering’, USA, describes what US automobile engineers are doing to beat the problem.

The purpose of this study is to study the pitting caused by Ca-Al-O-S composite inclusions of low-alloy steel in 3 Wt.% NaCl solution and 0.01M NaHSO3 solution.

The corrosion in 0.01M NaHSO3 was much weaker than in 3 Wt.% NaCl 3D display of the pitting formation and development process that has been calculated using scanning electron microscope (SEM) and laser scanning confocal microscopy (LSCM). In addition, a corrosion mechanism of pitting formation by galvanic interaction of composite inclusion and base metal has been proposed.

Results show that in immersion test, metal base around inclusions was dissolved due to corrosion. Corrosion on the metal base closer to inclusions was more severe.

A corrosion mechanism of pitting formation by galvanic interaction of composite inclusion and base metal has been proposed.

The application of cathodic protection to buried, submerged or immersed metals has become of increasing importance during the last three decades to civil, structural, mechanical, marine and chemical engineers. Although, at the moment, it has only a limited application for the protection of chemical plant, the method may become more important in this field in the future. The principles of cathodic protection, its applications, advantages and limitations and some of its present uses in protecting chemical plant are considered in the following article.

The purpose of this paper is to understand corrosion behavior of different Cu‐containing Al‐Si‐x% Cu alloys (x: 1 wt% Cu, 2 wt% Cu, 3 wt% Cu, 4 wt% Cu, and 5 wt% Cu) in 0.1 M HCl and 0.1 M H₂SO₄.

Potentiodynamic, chronoamperometric and impedance measurements were applied to specimens to obtain their electrochemical characteristics. For the long‐term analyses, hydrogen evolution with immersion time (V‐t) was measured. The corroded surfaces of the alloys were investigated using scanning electron microscopy (SEM) to understand the corrosion mechanism.

All experimental investigations showed that the corrosion resistance of alloys increased with increasing Cu content in the alloys.

Cu‐containing aluminum alloys are age‐hardenable alloys. The corrosion behaviour of these alloys can be changed by heat treatment. Corrosion test results for the heat treated and aged alloys will be discussed in another study.

Al‐Si‐Cu alloys are widely used in the automobile industry and the corrosion behaviour of these alloys has a great importance on the service life of these materials. Understanding the effect of copper and the corrosion mechanism of these alloys will be helpful in predicting and prolonging the service life of these materials.

To study the corrosion behavior of pipeline steel in coastal areas, a tidal seawater macro-cell corrosion device was built using a cycle soaking tank and a macro-cell corrosion facility to simulate the corrosion behavior of pipeline steel in a simulated coastal environment (dry and wet alternations during seawater-soil corrosion macro-cell processes).

The corrosion behaviors were studied via the weight loss method, electrochemical methods and morphological observations on corrosion.

The results show that during the initial stage of tidal seawater/soil macro-cell corrosion process of the X65 steel, the working electrode on the seawater side is the anode of the macro-battery. As corrosion progresses, the anode and the cathode of the macro-battery become inverted. As the area ratio and the dry – wet ratio increase, the time of anode and cathode inversion shortens. Galvanic current density decreases as the dry – wet ratio increases and increases as the area ratio increases. The corrosion process of macro-cell is affected by the reversal of anode and cathode. After the reversal of anode and cathode, the corrosion rate is mainly controlled by dry – wet alternating corrosion.

The corrosion behavior of a pipeline steel in a coastal environment was studied using a tidal seawater macro-cell corrosion device. The synergism effect between the tidal seawater and seawater-soil macro-cell on corrosion behavior was clarified.

The purpose of this study is to explore the mechanism of branch pits and tunnels formation and increase the specific surface area and capacitance of anode Al foil for high voltage electrolytic capacitor by D.C. etching in acidic solution and neutral.

Al foil was first D.C. etched in HCl-H₂SO₄ mixed acidic solution to form main tunnels perpendicular to the Al surface, and then D.C. etched in neutral NaCl solution including 0.5 per cent C₆H₈O₇ and Cu(NO₃)₂ with different concentration to form branch tunnels normal to Al surface. Between two etching, Cu nuclei were electroless deposited on the interior surface of main tunnels by natural occluded corrosion cell effect to form micro Cu-Al galvanic local cells. The effects of electroless deposited Cu nuclei on cross-section etching morphologies and electrochemical behavior of Al foil was investigated with SEM, polarization curve and electrochemical impedance spectroscopy (EIS).

The results show that sub branch tunnels can form along the main tunnels owing to the formation of Cu-Al micro-batteries, in which Cu is cathode and Al is anode. With increase in Cu(NO₃)₂ concentration, more Cu nuclei can be electroless deposited and serve as the favorable sites for branch tunnel initiation along the whole length of main tunnels, leading to enhancement in specific capacitance of anode Al foil.

Cu nuclei were electroless deposited on the interior surface of main tunnels by natural occluded corrosion cell effect to form micro Cu-Al galvanic local cells, which can serve as the favorable sites for branch tunnel initiation along the main tunnels to enhance specific capacitance of anode Al foil.

This study aims to propose a simple experimental method to distinguish the galvanic corrosion, crevice corrosion and self-corrosion in metal/carbon fiber reinforced polymer (CFRP) joints.

The corrosion behaviors of four different galvanic couples, whose anodes were Zn-coated DP590 steel and Al 6022, and cathodes were two kinds of CFRP, were investigated in immersion and GMW14872 cyclic conditions.

The results showed that the galvanic corrosion caused by direct contact between CFRP and metals was more serious than that caused by the jointing bolts. The corrosion damage caused by crevice corrosion was severer than that caused by galvanic corrosion. Self-corrosion was also significant, particularly under the cyclic salt spray condition.

Cyclic salt spray test may more reliably simulate the galvanic corrosion of a joint in industrial service environments, and real corrosion damage may be underestimated by a galvanic current measurement.

A deeper understanding of different corrosion mechanisms involved in CFRP/metal joints under different service conditions in industry has been given.

It is now increasingly recognised that surface coating technology offers production engineers and designers significant opportunities to optimise the use of raw materials. Due to the forecasted shortage in a number of engineering metals, such as zinc, mercury, tin … etc., surface coating technology offers the most attractive finishing process, with material conservations. Suitable bulk materials may be selected for cost or structural reasons, whilst surface coating materials are chosen to meet specific surface properties, such as wear resistance, protection against corrosion, surface thermal and electrical conductance, optical reflections and decorative features. The recent design trends towards higher speeds, minimum airplane weight and maximum load capacity, however, encourage the use of light weight titanium fasteners in airplane aluminium alloy structures. This creates a serious galvanic corrosion problem to airplane skin sheets. In the following a new surface coating technique which is recently recognised as a growth of a new technology is applied to the problem of galvanic corrosion in air frame structures. The application of aluminium coatings for the protection of airplane skin sheets and fasteners against the galvanic corrosion in local environments is investigated. Both polarisation and galvanic tests are used for the evaluation of the potential of the new surface coatings. Furthermore both sodium chloride and sulphur dioxide electrolytes are used to simulate sea water and jet exhaust environments. Electrolyte saturation with either air or nitrogen are considered to compensate for the presence and lack of oxygen at different environments. It is concluded that the strong adhesion and the extensive graded interfaces of the ion plated films are responsible for the good protection of coated metallic couples.

The purpose of this paper is to study, under laboratory conditions, the corrosive effect of sulfur dioxide (SO₂) present in the atmosphere of urban and industrial areas on various construction materials.

Iron, copper and brass metals were exposed to SO₂ gas at different relative humidities that were obtained using analytical grade glycerol and water mixtures. The corrosion rates (mdd) of the samples were determined over 120 h using the weight‐loss method under fixed relative humidity (rH) conditions. The change of galvanic current was measured as a function of exposure time over 196 h. Nyquist diagrams were obtained in 10⁻³ M Na₂SO₄ solutions with a pH value of 7.2, which was assumed to correspond to 100 percent rH conditions.

The obtained data showed that the corrosion rate of the studied metals increased with increasing rH. The corrosion rate of the metals decreased with exposure time, due to accumulation of corrosion products over the surface of the metals. However, the surface films of corrosion products on the metal surfaces were not stable and the corrosion rate increased again with time when the surface film disappeared.

The atmospheric corrosion of the industrial materials is dependent upon the rH and SO₂ concentration. The corrosive effect of SO₂ present in the atmosphere of urban and industrial areas on various construction materials can be tested under laboratory conditions.

The effects of SO₂ and NH₃ on the atmospheric corrosion of galvanized iron and the effect of rH on the atmospheric corrosion of defective organic coating materials were reported in literature. In this study, the corrosive effect of SO₂ present in the atmosphere of urban and industrial areas on various construction materials (iron, copper and brass) under laboratory conditions was studied.