J.B. Johnson

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.

The Influence of Combustion-derived pollutants on limestone deterioration: 2. The wet deposition of pollutant species

This study concerns quantification of the relative chemical degradation effects of dry deposition of combustion-derived atmospheric pollutants, HCl, SO2 and NO2, on Portland and Monks Park limestones, employing laboratory exposure chambers for periods of 30 days. Using presentation rates up to 40 × 10−4 μgcm−2s−1, the pollutants were assessed individually and in various combinations, along with ozone as oxidant, at 84% RH with dry or water-wetted surfaces. The degradation was followed by analysis of exposed stone, for Cl−, SO42− and NO3−, and of run-off solution for Ca2+, in addition to the anions. From these data, the total calcium released from limestone to reaction ions and products and the percentages of each pollutant reacted (and so the deposition velocity (VD)) in each exposure regime were calculated. HCl acted independently of the presence of other pollutants, showed 40 and 100% reaction (mass transport control) and VD values of 2.2 and 5.3 mm s−1, at 84% RH and on wetted surfaces, respectively. The reactivity of SO2 rose from 2.5 to 35%, and VD from 1 to 2.9 mm s−1, from conditions of 84% RH to those with the presence of water, NO2 and O3. Similarly, the reactivity of NO2 rose from 3 to 40% at 84% RH in the presence of SO2 and O3, with a VD at 3 mm s−1, but decreased in the presence of run-off solution. A limited study was performed on smoke-coated and naturally exposed, reaction-product-coated, samples. In addition to obvious factors such as deposited mass, chemical constitution and transfer, smoke particle size distribution should also be considered in any possible action on limestone. Concerning naturally exposed samples, non-wetted surface reaction products tended to reduce the attack of pollutants, but had no resisting effect when wetted.

The effect of fly-ash particulates on the atmospheric corrosion of zinc and mild steel

The atmospheric corrosion of zinc and mild steel was investigated in the laboratory at relative humidities of 65%, 80% and 90% in unpolluted atmospheres and at a relative humidity of 80% in atmospheres containing HCl and SO2 pollutant gases. In order to investigate the effect of atmospheric particulates on corrosion rates, metal samples were contaminated with three coal and three oil fly-ashes from different industrial sources. Control specimens were either uncontaminated or contaminated with small glass beads of similar size to the fly-ashes < 45 μm. In unpolluted atmospheres, particulate contamination increased corrosion in approximate proportion to the quantity of leachable ionic species present in the fly-ash. Additionally, glass beads slightly increased corrosion rates probably due to differential aeration effects and an increased local time-of-wetness in the vicinity of the beads. In polluted environments, the corrosion rates of the specimens increased and the additional effect of fly-ash contamination on the corrosion rates was consequently decreased in proportion to the presentation rate of pollutant. There was no significant additional increase in corrosion rates with SO2 pollutant and fly-ash contamination, indicating that effects due to catalytic oxidation of SO2 to sulphuric acid or sulphates were not significant. Overall, this study provides strong evidence that the atmospheric corrosion rates of metals are dependent on the conductance of the thin-film surface electrolyte and that the first-order effect of contaminant particles is to increase solution conductance and hence corrosion rates.

Corrosion in smoke, hydrocarbon and SO2 polluted atmospheres—I. General behaviour of iron

Abstract Iron foil specimens have been exposed for up to 145 h in a modified industrial atmosphere test chamber, to a control air-based environment at 298 K with a relative humidity of 85% and a CO 2 concentration of 5500 ppm. Additions of SO 2 (3.5 ppm), ethane, ethylene or acetylene (8 ppm) and smoke produced by burning kerosene (up to 560 μg m −3 average concentration) have been made singly and in appropriate combinations, so the separate and combined effects could be examined subsequently by weight change, metallography, SEM/EDX etc. The smoke aerosol was characterized by size and shape using TEM, and its influence on the conductivity and pH of water assessed by appropriate methods. The many results are analysed in terms of considerations such as sorption, catalytic activity at active complexes on smoke platelet surfaces and the interaction with moisture and free radical chain oxidation mechanisms, in an attempt to bridge the gap between simple observation and the clearly subtle chemical reactions which can control such atmospheric corrosion.

Stone degradation due to wet deposition of pollutants

As part of an overall programme of study upon the action of atmospheric pollutants on building stones, the degradation of limestones has been measured by dissolution in various solutions with different anions, in simulation of attack by wet deposition of salts found in acid rain. In water, CO2 plays a significant role in increasing material losses from limestone. Solution volume and permitted reaction time are also important factors. With pH decrease the CO2 role becomes less significant and the calcium entering solution is then controlled by the pollutant (acid)/stone(base) reaction. In neutral salt solutions reactions are determined by the buffering capacity of the solution, limited by solubility criteria, and modified by common ion effects (reducing solubility) and by shielding ions (increasing solubility). Ammonium salts have an intermediate position but act predominantly through the pH effect.

The degradation of coupled stones by wet deposition processes

Abstract An investigation has been made of the influence of wet deposition of simulated acid rain, with subsequent dry/wet cycling, on the degradation of limestone, sandstone, marble and granite, interactively coupled by overlapping or butt joints to simulate architectural detail, and sloping at an angle so that run-off from the upper stone drains on to the lower one. Degradation was monitored by pH change and analysis of run-off solution and soluble extracts from the exposed stones, together with EPMA, SEM/EDX and X-ray diffraction examination of petrographic sections. Overlapping coupled stones show greater degradation of the lower stone for all stones, but particularly when it is granite and the upper stone is limestone. This is partly due to a longer time of wetness at the interface and closely adjacent areas (meniscus effect), and partly due to the deposition of ions from the upper stone and the buildup of damaging degradation products (mainly gypsum) on the lower stone and interaction with it. Butt-jointed couples scarcely show the effect due to no significant meniscus effect. This method of laboratory simulation for stones shaped and interactively coupled to reproduce protruding and re-entrant surfaces in typical architectural combinations, so that drainage from one to another occurs, can yield results similar to those found in practice, and be used to rank various stone combinations.

A laboratory simulation of degradation of leinster granite by dry and wet deposition processes

Abstract The accelerating effect of pollutant gases and acids on the degradation of ‘fresh” Leinster granite has been simulated in laboratory-based dry deposition and wet deposition exposure chambers. Granite degradation has been followed by weight change, stone extract and sectional analyses, pH change and cation content of the run-off solution. General deterioration of the granite structure is revealed by petrographic analyses to occur by a series of mineralogical tranformations, of which kaolinization and seritization are the most clearly visible. Microcracking, predominantly in the alkali/microcline and plagioclase feldspar crystals, is evident on the exposed side, especially in simulated acid rain solutions but not in deionized water. Ion exchange of H + for Ca 2+ , allied with mineralogical transformation and salt crystallization in pores, evidently promote the cracking. Calcite and gypsum deposition in intergranular regions and cracks are also observed and there is evidence of the removal and oxidation of iron species on the mineral surface.

The influence of an isobutane-SO2 pollutant system on the earlier stages of the atmospheric corrosion of metals

Abstract Whilst much research upon atmospheric corrosion has involved SO2, the pollutant smoke aerosol and associated sorbed hydrocarbons have been largely ignored. As a preliminary study, the present work concerns the corrosivity of air-based atmospheres containing 50 vpm isobutane and/or 3.5 ppm SO2 at 78 % r.h. It was found that, relative to a clean air atmosphere, isobutane inhibited the corrosion of iron and nickel whilst having rather less effect upon zinc, aluminium and copper. isobutane inhibited the corrosivity of SO2 to iron, whilst conversely enhancing the corrosivity of SO2 to aluminium, copper, nickel and zinc. In order to account for these results, proposed explanations involve the molecular dipole polarization of isobutane, its adsorption and coordination association at anodic and cathodic sites, affecting electrochemical reactions both directly and indirectly by mass transfer limitations, and its interference with maintenance repair reactions on surface layers.

Short-term atmospheric corrosion of mild steel at two weather and pollution monitored sites

Results are recorded of topographical details, weight loss and corrosion product analysis for short-term atmospheric exposure of mild steel, in relation to weather and pollutant factors. Various structured corrosion features, e.g. doughnut-like, were noted. The major corrosion product found was γ-Fe2O3,H2O, with some α-Fe2O3,H2O and α-FeOOH. Wet initial conditions of exposure and subsequent high levels of humidity, were found to have the dominant overriding effect in promoting corrosion losses. Smoke was shown to be capable of exerting a very strong influence upon the effective corrosivity of atmospheric sulphur dioxide.

Corrosion in smoke, hydrocarbon and SO2 polluted atmospheres—II. Mechanistic implications for iron from surface analytical and allied techniques

Abstract AES and XPS have been applied to inspect the early stage corrosion products (4 h for AES and 24 h for XPS) formed on electropolished iron specimens after exposure to smoke, certain hydrocarbon gases and SO2 in a laboratory controlled atmosphere test cabinet. XPS studies have also been used to characterize the reactivity of oxide-free and hydroxide/oxide iron surfaces towards humid air and SO2. The reactivity of iron hydroxide/oxide surfaces towards ethane and ethylene has been investigated by mass spectroscopy. The results indicated that SO2 is directly and rapidly corrosive towards iron. This contrasted with the initial interactions with smoke and hydrocarbons, which apparently are not immediately so corrosive. A clean oxide-free iron surface reacted strongly with either humid air or SO2 but the hydroxide/oxide surface was less reactive towards SO2. Iron hydroxide/oxide surfaces may have caused fragmentation of C2H6 to CH3- radicals and some fragmentation of C2H4 to CH2-, although molecular chemisorption of C2H4 was more prevalent. The results obtained provide mechanistic interpretations for the observed corrosion enhancement and modifying actions induced by smoke and hydrocarbons reported in an earlier paper in this series.