S.B. Lyon

Corrosion control of galvanized steel using a phosphate/calcium ion inhibitor mixture

The corrosion inhibition of galvanized steel was studied in artificial acid rain solution using extracts of pigments normally used in organic coatings for corrosion control. It was established that a combination of zinc phosphate/molybdate and calcium ion exchange silica has a significant synergetic anticorrosion effect in the acid rain solution compared to the pigments used alone. Further, the charge transfer resistance of galvanized steel in acid rain solution saturated by the above pigment blend approaches that of strontium chromate in artificial acid rain solution. Use of the pigment blend was found to lead to development of a protective film, which is thought to be a complex mixture of calcium phosphates and zinc phosphate.

Phenomenological modelling of atmospheric corrosion using an artificial neural network

World-wide data for the atmospheric corrosion of steel and zinc were used to train and test neural networks. The cross validity technique was employed as the criterion to stop training. The statistical performance of the neural network was expressed as an average of five training and testing results. Multiple correlation coefficients showed that the neural network could account for about 70% of the variance in the corrosion data for steel and zinc. The testing results showed that predictions of corrosion data not included in the original training were close to practical data. Sensitivity analysis also demonstrated the effects of sulphur dioxide, chloride and exposure time on atmospheric corrosion in specific environments.

Effect of water and its activity on transport properties of glass/epoxy particulate composites

Abstract Silane coated glass microspheres embedded in an epoxy polymer matrix have been employed as a model system to investigate water sorption at three water activities. Composites containing soda-lime-silica (A) glass in the range 0–25% by volume were tested in deionized water, saturated NaCl and saturated MgCl2, at 40°C. Increase in water activity led to a decrease in the effective water diffusivity due to trapping, especially at interfaces ruptured by osmotic pressure. Higher water activities favoured interfacial water transport, whereas lower water activities favoured water transport through the bulk of the polymer. Electrical resistance and water permeation tests suggested very rapid preferential water transport through areas of low cross-link density, even in the pure polymer, enhanced by the introduction of interfaces. Effective diffusivity calculations during water absorption and desorption, together with electrical resistance measurements and water uptake measurements in saturated NaCl, suggested the formation of a pecolating liquid channel network for composites containing more than 15% glass. Electrical resistance measurements using silver-coated A-glass/epoxy composites located the percolation threshold at 16% glass. Similar water sorption effects were observed with uncoated A-glass filled epoxy composites.

The corrosion of iron and zinc by atmospheric hydrogen chloride

Abstract Laboratory-based measurements of the corrosion of iron and zinc in HCl have been carried out under a range of conditions simulating natural atmospheres. The corrosion rate of zinc did not significantly increase upon exposure to HCl at presentation rates typical of the highest found in an urban area (2.5 × 10 −6 mg cm −2 s −1 ). This is explained by the formation of protective basic zinc chloride by reaction with pre-existing zinc hydroxide. At higher levels of pollutant (representative of episodic industrial atmospheres) reaction of HCl with zinc hydroxide proceeds further to form soluble zinc chloride. Consequently, the protective ability of the corrosion product is lost. Under these conditions, corrosion rate of zinc is controlled by the availability of HCl at the metal surface. In contrast, the corrosion rate of mild steel samples exposed to HCl presentation rates typical of the highest encountered in an urban atmosphere was 18 times that found in an unpolluted atmosphere. Increasing the HCl presentation rate further did not result in a significant increase in corrosion rate. These results are explained by the reaction of HCl at the metal surface and the subsequent formation of FeCl 2 , which is oxidized to FeO(OH), liberating HCl which can initiate further corrosion. The reaction sequence forms a cycle and is, therefore, apparently independent of incoming HCl.

Investigating Localized Degradation of Organic Coatings Comparison of Electrochemical Impedance Spectroscopy with Local Electrochemical Impedance Spectroscopy

The degrdn. of polyester coil-coated galvanized steel was compared using both conventional (macroscopic) and localized electrochem. impedance techniques on the same specimen and within a time interval (hours) much shorter than the total immersion period (days). Specimens contg. a central 250 mm laser-ablated defect in the org. coating layer were immersed in a 10 mM NaCl soln. for up to 30 days. The local multi-frequency impedance was detd. by placing a novel impedance probe, either directly above the coating defect or above an area of intact coating. In addn., single frequency impedance mapping of the specimen surface was carried out at 1 kHz and compared with optical microscopy of the surface. The results demonstrate clearly that macroscopic electrochem. impedance provides a surface-averaged measurement of the properties of the coating, plus any defects, and that where several time consts. are apparent, they are not uniquely separable into phys. processes. Thus, macroscopic impedance spectra convolute the sep. responses of the coating and defect together. However, local electrochem. impedance can effectively sep. the local properties of the org. coating from the local electrochem. behavior at a coating defect. [on SciFinder (R)]

Inhibition of cut edge corrosion of coil-coated architectural cladding

This study has investigated the solubility of inhibiting pigments and also the leaching behaviour of corrosion inhibitors from a polyester binder in an acid rain solution. This study has relevance to the control of cut-edge corrosion by inhibitor pigments, which are incorporated into the primer coatings of coil-coated architectural cladding. The concentrations of leachate solutions in tubes glued to the top surface of primer-coated galvanised steel specimens were measured using inductively coupled plasma emission spectroscopy (ICPES) for a range of commercial inhibitors. In addition, the saturated solubility was measured using ICPES. It was found that the leaching rate, rather than the saturated solubility, controls the amount of inhibitor which is available for corrosion control at cut edges and in a coating defect area. These results are compared to impedance testing of polyester-coated galvanised steel specimens coated with the same primers.

A surface analytical study of functionalised mild steel for adhesion promotion of organic coatings

Abstract Organosilane compounds are among the most extensively studied compounds in the field of adhesion enhancement of organic coatings on an inorganic substrate. Much of its performance has been related to its ability to form a “bridging effect” at the interfacial region. Nevertheless in order for this to happen, the chemical and physical state of the compound after application on the substrate is very important. Variations in molecular orientation and the chemical state of the compound functionality have been known to give unexpected results on its ability to enhance adhesion of certain organic coatings to a substrate. In this work the nature of 3-aminopropyltriethoxysilane applied on mild steel substrate was studied using X-ray photoelectron and infra-red spectroscopies. It was found that when this compound was applied under common conditions, the amine functional end of the compound appears to bond to the metallic substrate, and that it may be present in the form of a non-protonated amino group or hydrogen-bonded amino group. This “wrong-way round” adsorption is shown to profoundly affect organic coating performance.

The electrochemistry of iron, zinc and copper in thin layer electrolytes

Abstract An electrochemical cell, suitable for the study of electrochemistry in thin-layer electrolytes, has been designed and produced. Comprehensive electrochemical research on this electrochemical cell under various electrolyte thicknesses (100–1132 μm) has been carried out using a d.c. (potentiodynamic polarization) electrochemical technique. By this method, it is hoped to elucidate information concerning the mechanisms of reactions occurring in thin-layer electrolytes and to what degree the results obtained can be extended to research into atmospheric-corrosion phenomena.

The long-term corrosion of aluminium in alkaline media

Abstract The corrosion of aluminium in sodium hydroxide solutions in the pH range 8–12 was studied at 30 and 60°C for up to 80 days with control of pH ensured by frequent change of solution. The corrosion was found to be controlled by competitive processes involving film growth and dissolution. The film formed consisted of a compact inner barrier layer, adjacent to the aluminium substrate, and an outer crystalline layer of total thickness in the range 5–10 μm. The average corrosion rates after 40 days were relatively low, g m −2 y −1 , below pH 10 at both temperatures but increased rapidly under more alkaline conditions. Additions of Cl − ion at pH 10 had no significant effect on the corrosion rate at 30°C but reduced the rate to one-third of its previous value at 60°C. Below pH 10, the specimens developed a pit-like morphology under the compact layer. At pH 11 cavities developed in the crystalline film allowing greater solution access and more rapid corrosion. At pH 12, occasional cavities developed into relatively large features resembling pits, resulting in rapid disintegration and dissolution of the specimen.

An electrochemical study of the atmospheric corrosion of mild steel-I. Experimental method

A conventional three-electrode measurement system using a Luggin capillary was successfully developed to study the corrosion of metals under thin films of electrolyte. The experimental arrangement also permitted condensation and evaporation of electrolyte directly on to the metal surface from a gas ambient of controlled composition and relative humidity. This arrangement was intended to allow conditions similar to those of atmospheric corrosion. Automatic resistive compensation in the electrolyte was carried out using a current interrupting potentiostat. Using this apparatus, good potentiodynamic data could be obtained with an electrolyte film thickness of 300 μm using an electrolyte of 0.2 M Na2SO4. Under these conditions, the cathodic kinetics were clearly controlled by the ambient O2 concentration. The oxygen reduction rate was found to increase significantly during evaporation of an electrolyte film which is consistent with increased oxygen transport across the reducing film thickness. However, measurement of the electrolyte film resistance showed a rising resistance with decreasing film thickness. This is consistent with ion transport in the plane of the electrolyte film parallel to the metal substrate.