Corrosion Engineering,Science and Technology: The International Journal of Corrosion Processes and Corrosion Control

Several variables have been studied in four factorial experiments in order to determine their influence on the rate of de-tinning of cans containing tomato products, as follows: net weight; filling temperature; initial nitrate content; Brix value; pH; tinplate steel annealing; bodycorrugations; exposed steel; and some anions. Nitrates and net weight (i.e. residual internal air) were found to be the major factors influencing de-tinning; also, their effects were found to be independent and additive. In 500 g cans stored at 20° C for 24 months, the increase of dissolvedtin is proportional to nitrates and the net weight according to the estimated ratios:

Dissolved Tin/Initial Nitrate Content = 117 mg kg⁻¹/20 mg kg⁻¹

and

Dissolved Tin/Net Weight Change = −47 mg kg⁻¹/20 g

Technological considerationsabout the importance of Brix are discussed, and the predicted distributions of dissolved tin after 24 months are calculated for a simulated batch of 100 000 cans. According this model, the levels of 150, 200, 250 mg kg⁻¹ of tin content would be exceeded by 64%, 25%, and 4%, respectively,of cans filled with high Brix tomato products (6.5° Bx); in the case of the less aggressive ‘natural Brix’ products (5.0° Bx), the same levels of tin content would be exceeded by 42%, 6%, and 0.5% of the cans.

This paper is the sixth paper outlining a holistic model of atmospheric corrosion. Previous papers outline how airborne salinity could be estimated at any given location and how the deposition onto a surface could be modelled. This paper investigates the initial reactions of raindrops or deposited marine aerosols on metal surfaces (zinc, galvanised steel, aluminium and gold). It combines short term field exposures with a series of controlled laboratory experiments. Surface changes to the metal upon field exposure are characterised by the nature and distribution of corrosion nodules, oxide films and retained salts. Eleven distinct forms of surface change are observed. The conditions promoting each pattern of surface changes were estimated by comparison with laboratory tests with a variety of deposition modes. Laboratory tests included deposition of saline droplets in three size ranges. A strategy to use the results in a holistic model of corrosion is proposed.

The corrosion of reinforcing bars (rebars) is the main cause of premature failure of reinforced concrete structures. The ingress of chlorides and the carbonation process that leads to the neutralisation of the alkaline pore solution are among the key phenomena promoting such corrosion. This paper deals with the effectiveness of commercial corrosion inhibitors to prevent the corrosion of reinforcement in concrete. Three organic inhibitors and, for comparison, a calcium nitrite based product, were added to fresh concrete in the dosages suggested by producers. Experimental tests were carried out both in chloride contaminated concrete (produced by adding chlorides directly to the mixture or by penetrating hardened concrete using ponding cycles) and in carbonated concrete. In order to study the effectiveness of the inhibitors on the initiation of corrosion, measurements were made over two years of two electrochemical parameters, the free corrosion potential and the polarisation resistance. These are discussed and compared with results obtained for concrete specimens without inhibitors. One of the organic corrosion inhibitors and the nitrite based product both delayed the initiation of corrosion.

The wear and corrosion properties of SiC_p/graphite reinforced copper metal matrix composites (MMCs) fabricated by hot pressing have been evaluated. Electron microscopic studies were conducted to clarify the micromechanisms of wear and corrosion. The experimental resultsindicate that the hardness and wear loss increase with increasing SiC_p content but decrease with increasing graphite content. In particular, the wear loss decreases with increasing graphite up to a content of 5 vol.-% then remains approximately constant as the graphite content isfurther increased to 7·5 vol.-%. The material porosity of the composites increased with the content of foreign particles (SiC_p and/or graphite).

The composites were immersed in an aqueous solution of 3.5 wt.-%NaCl at pH 6·7 for potentiodynamic and corrosion ratemeasurements. Pure copper exhibited the best corrosion resistance. The Cu/SiC_p composite showed lower resistance, and the Cu/SiC_p/graphite composite had the lowest resistance to corrosion. The poor corrosive properties of the composites were the result of galvanic andcrevice corrosion, which were influenced by residual stress/strain and differences in electrochemical potential between the additions (SiC_p and/or graphite) and the copper matrix. Furthermore, the weight loss due to corrosion increased with increasing SiC_p and graphitecontent.

Three nickel base alloys strengthened by different hardening effects were investigated by thermogravimetry in air under isothermal conditions. The alloys investigated were γ′-Ni₃ (Al, Ti)-hardening alloy 80A (75Ni, 21Cr, 2·5Al, 1·7Ti, DIN No. 2·4952),solid solution hardened alloy C22 (59Ni, 21Cr, 13Mo, 3·5 Fe, 2·8W, DIN No. 2·4602) and a new high nitrogen containing and nitride hardening alloy N (61Ni, 27Cr, 10W, 1·4Ti, 0.2N). Tests were conducted in air between 900 and 1100° C for 48 h. Parabolic oxidationrates were determined and the formation of the oxide layer was investigated by optical microscopy and SEM. Oxidation data showed that the hardening mechanism has almost no influence on the oxidation kinetics. All of the alloys investigated formed chromia layers. After initial transient stateoxidation, the kinetics followed a parabolic law. Alloy 80A had the highest oxidation rate of the investigated alloys, which is attributed first to its lower chromium content and second to the formation of chromium carbides. At grain boundaries, internal oxidation, mainly of aluminium andtitanium, took place. The Al and Ti contents of alloy 80A were too low for the formation of a protective inner oxide layer of one of the two elements to take place. Alloy C22 showed the best resistance to oxidation since its chromium content of 21% is close to that for the minimum in the kineticsof oxide formation that has been found for binary Ni–Cr alloys. Additionally, there were no chromium rich precipitates to shift this chromium content to values that would result in higher oxidation rates. The nitride-containing alloy N contained a higher chromium content of 26%, whichled to a higher oxidation rate than that for alloy C22. A certain amount of inner oxidation took place, especially at coarse Cr₂N precipitates. Conclusions are presented about the optimised chemical composition of chromia laye-forming nickel base alloys for minimised oxidationrate.

A monitoring method for the corrosion rate of metal based on the coulostatic principle and a new mathematical model for the evaluation of Tafel coefficients from coulostatic experimental data are presented in this paper. Different kinds of equivalent circuit models involving R(CR(RL)) and R(RC) have been applied to examine the characteristics of the response curves to coulostatic and AC perturbations. In addition, weight loss and electrochemical impedance spectroscopy (EIS) experiments on Q235 mild steel in 0.5 M H₂SO₄ solutions with and without thiourea as inhibitor have been used to verify the results from the coulostatic method. It was found that the coulostatic technique can not only determine values of R_p and C_d quickly, but also the Tafel coefficients in the acidic solution with and without inhibitor. Moreover, since coulostatic measurements can always assume R_p equal to R_t in this system, the corrosion rate calculated by the coulostatic method is free of confusion between R_p and R_t. This facilitates the rapid calculation of corrosion current density based on the Geary–Stern equation, and provides a simple procedure for the evaluation of corrosion and corrosion inhibitors.

During service, steel ropes can suffer from corrosion, which can reduce their mechanical properties. This degradation is particularly important in the case of structural ropes, or barriers, for snow and rockfall protection because it is not easy to detect any significant degradation over time due to the locations of these structures, which are often difficult to reach. Moreover, rockfall barriers are frequently used in marine environments to reduce the instability of rocks and cliffs, where corrosive attack by chlorides can be very dangerous. This study has characterised the effects on corrosion of the geometrical configurations of the strands and wires in the ropes, and the benefits of zinc and zinc alloy protective coatings on the wires for reducing the degradation of structural ropes. The aim of the research was to obtain information useful for the design of high durability barriers and the selection of an appropriate rope and coating. An approach based on an electrochemical testing has been compared with one based on classical tests involving the exposure of ropes in aggressive environments (salt spray, humidity chamber, etc.) in order to find an electrochemical method able to distinguish the performance of different materials as well as to monitor the corrosion degradation of barriers in the field.

Using electrochemical techniques it was possible to quantify the corrosion rate and demonstrate the better behaviour of ropes coated with zinc–aluminum alloys in comparison with pure zinc. Moreover it was possible to evaluate the actual reactive area, and therefore the protective geometry effect when comparing two ropes with different geometries. This approach should be extended to further kinds of ropes in order to improve basic corrosion understanding of this important structural component.

A novel dynamic electrochemical impedance spectroscopy (DEIS) technique has been developed that offers the possibility to determine impedance characteristics in a time domain during the rupture of the protective passive layer. This ability is associated with the analysis methodologyof the technique, which enables selective time–frequency analysis of the measurement data. This paper proposes that the DEIS technique allows the acquisition of instantaneous changes during the passive layer cracking process, which is strongly related to the initiation stage of stresscorrosion cracking. Moreover, the principles of application of the novel method in such investigations are discussed.

Bacterial evidence was found in the biofilm deposited on the surface of an API X52 steel coupon following exposure to flowing seawater within a pipeline for 60 days. Extensive corrosion pitting was observed on the steel surface after removing the biofilm. Several types of anaerobic and aerobic bacteria were isolated from the biofilm. The anaerobic bacteria were cultured and grown in API RP 38 culture medium and the aerobic bacteria were grown in a general heterotrophic culture medium. The morphological and chemical characteristics of the bacteria were determined. Subsequently, separate specimens of API X52 pipeline steel were inoculated with each bacterial strain in under laboratory conditions. The specimen exposed to the anaerobic bacteria showed several micropits of different sizes on its surface while that exposed to the aerobic bacteria showed freedom from surface pits.