Atmospheric corrosion of zinc induced by runoff

Atmospheric corrosion and runoff of zinc were investigated during two years in humid tropical climate on hot dip galvanized steel and zinc samples. The high zinc mass loss (14.70 g m⁻²) is induced by the intensive zinc release (12.40 g m⁻²). No corrosion phase containing chloride was detected on the zinc surface, while a variety of sulfates not dissolved by rains reveals the sensitivity of zinc to SO₂ pollutant. However, two chloride-containing corrosion products were detected on the galvanized steel. Exponential equation is proposed that fits well the experimental data for zinc mass loss induced by runoff process as a function of the time of wetness. The formula gives possibility to predict the mass loss even before a steady state in the corrosion process has been reached. This equation can converge to a Benarie lineal function (C = At_w), when the coefficient b = 1 for the corrosion which is accelerated with the partial removal of the corrosion layer during the runoff phenomena.     

Zinc runoff from organically coated galvanised architectural steel

The metallic runoff from the exposed cut-edges of galvanised steel substrates (zinc and an experimental zinc/aluminium alloy) over coated with organic coatings have been quantified over a 30-month period of external exposure. The organic coatings comprised a 200 μm PVC plastisol coating on one side and a 15 μm polyester coating on the other with compatible chromate based pre-treatments and primers. A novel method of increasing cut-edge length per unit area has been developed in which samples are punched with a regular array of holes. This increases the available cut-edge length per unit area by three orders of magnitude over a standard roofing panel. The only measurable leachates detected (>0.5 ppb (part per billion)) over the exposure period have been zinc and iron. All the samples showed significant reduction in zinc runoff upon prolonged exposure with zinc leaching concentrations over a three-month sampling period typically falling from a maximum 1-2 ppm (part per million) upon initial exposure to 0.05-0.1 ppm after 12 months exposure in the harshest industrial and marine sites. Metal ion runoff has been lowest in rural weathering locations (typically 0.5 ppm in a three-month exposure period upon initial exposure falling to <0.05 ppm over the same time period after 12 months exposure).In general at the more severe marine and industrial sites the greatest metal runoff levels were detected from organically coated substrates made from experimental metallic coatings with a 4.95% aluminium, 0.05% magnesium and 95% zinc metallic coating with less runoff leaching from 99.85% zinc coatings. In the former substrate this is due to the presence of subsurface pro-eutectic zinc rich dendrites that are exposed at the cut-edge but are not present in the corrosion resistant aluminium rich surface of the material. In these cases zinc leaching is independent of whether the samples are over-coated with organic layers.     

Electrochemical impedance study on galvanized steel corrosion under cyclic wet-dry conditions--influence of time of wetness

Electrochemical impedance technique has been applied to study the corrosion behavior of galvanized steel under wet-dry cyclic conditions with various drying periods. The wet-dry cycles were carried out for the period of 336 h by exposure to alternate conditions of 1 h immersion in a 0.5 M NaCl solution and drying for various time periods (11, 7 and 3 h) at 298 K and 60% RH. During the wet-dry cycles, the polarization resistance, R_p, and solution resistance, R_s, were continuously monitored. The instantaneous corrosion rate of the coating was estimated from the obtained R_p⁻¹ and time of wetness was determined from the R_s values. The corrosion potential, E_corr, was also measured only during the immersion period of each wet-dry cycle. In all cases, the corrosion was accelerated by the wet-dry cycles in the early stage, and started to decrease at a certain cycle and finally became similar to that at the initial cycle. The underlying steel corrosion commenced after the corrosion rate started to decrease. The shorter drying period in each cycle led to higher amount of corrosion of the coating because the surface was under wet conditions for longer periods. On the other hand, time to red rust appearance due to occurrence of the underlying steel corrosion became shorter as the drying period increased, although the total amount of corrosion was smaller. The corrosion mechanism of substrate steel under various drying conditions has been discussed, the galvanic coupling effect being taken into account.     

Influence of drying temperature on the corrosion performance of chromate coatings on galvanized steel

The influence of drying temperature on the corrosion performance of chromate coatings on electro‐galvanized (EG) steel has been investigated using electrochemical impedance spectroscopy (EIS) and potentiodynamic measurements in 3.5% NaCl solutions. The chromate coatings were applied to the EG steel in a solution (pH 1.2) containing sodium dichromate and sulfuric acid at room temperature. The coatings were dried in an oven at three different temperatures: 60, 110 and 210°C. The surface of the chromate coatings was analyzed using atomic force microscopy (AFM) and scanning electron microscopy (SEM) combined with energy‐dispersive spectrometry (EDS). The results show that the drying temperature significantly affects the morphology of the chromate coatings and consequently affects their corrosion behavior. The chromate coatings dried at 110°C had few cracks and the lowest corrosion current. The chromate coatings dried at 60°C showed passivity. The EIS results show that the chromate coatings dried at 60°C has the largest impedance in a neutral 3.5% NaCl solution. Drying at higher temperature (210°C) degrades the chromate coatings by widening the cracks and reducing soluble Cr(VI) in the chromate layer. The favorable drying temperature for the chromate coatings on the EG steel is between 60 and 110°C.     

Seasonal variations in corrosion rate and runoff rate of copper roofs in an urban and a rural atmospheric environment

This paper summarizes the results from an extensive field exposure program implemented to study possible seasonal dependencies of copper corrosion rates and runoff rates. Two-year exposures in one urban and one rural environment were performed at four different starting seasons. An extensive multi-analytical approach was undertaken of all exposed samples.Seasonal differences in corrosion product formation was observed during the first month of exposure and attributed mainly to differences in relative humidity conditions. Seasonal differences in corrosion rate at the rural site could be discerned throughout the whole two-year exposure, again, mainly attributed to differences in relative humidity. No seasonal effect could be observed at the urban site indicating that other parameters influenced the corrosion kinetics at this site. While corrosion rates exhibit a continuous decrease with exposure time, the yearly runoff rates are independent of time. Depending on starting months the yearly copper runoff rates ranged from 1.1 to 1.7 g m⁻² y⁻¹ for the urban site, and from 0.6 to 1.0 g m⁻² y⁻¹ for the rural site. These seasonal variations were primarily attributed to differences in precipitation quantity and environmental characteristics. Runoff rates are significantly lower than corrosion rates as long as the adhering copper patina is growing with exposure time.A full risk assessment requires not only information on the total amount of copper in the runoff, but also on its chemical speciation. Under present conditions, 70–90% of all copper in runoff water collected immediately after leaving the surface is present as the most bioavailable form, the hydrated cupric ion, Cu(H₂O)₆²⁺.     

Synergistic corrosion protection for galvanized steel by phosphating and sodium silicate post-sealing

To improve the corrosion resistance of phosphate coatings, the phosphated hot-dip galvanized (HDG) sheets were post-sealed with sodium silicate (water glass) solutions. The morphology and chemical composition of the composite coatings was analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The effect of sodium silicate post-sealing treatment on the corrosion behaviors of phosphate coatings was investigated by neutral spray salt (NSS) tests and electrochemical measurements. The results show that after the silicate post-treatment the pores among zinc phosphate crystals are sealed with the films containing Si, P, O and Zn, leading to the formation of the continuous composite coatings on the surface of HDG steel. The corrosion resistance of the composite coatings depending on concentration of sodium silicate and post-sealing time is greatly improved by the silicate post-treatment. The optimum concentration of silicate and post-sealing time are 5 g/L and 10 min, respectively. Both the anodic and cathodic processes of zinc corrosion on the samples are suppressed conspicuously, and the synergistic protection effect of the single phosphate coatings and the single silicate films is evident. Moreover, the low frequency inductive loop in electrochemical impedance spectroscopy (EIS) is disappeared and the electrochemical impedance values are increased for more than one order of magnitude. The corrosion protection of the composite coatings is comparable to that provided by the chromic acid post-treatment.     

A comparative study of the corrosion protective properties of chromium and chromium free passivation methods

Commercially available passivation methods for white-rust protection of hot-dip galvanized steel have been investigated. The passivations were either based on trivalent chromium or chromium free. A chromate based conversion coating was used for reference. The treated panels were tested with regard to white rust protection and paintability. The surface chemistry of the conversion coatings was monitored with scanning Auger electron spectroscopy and X-ray photoelectron spectroscopy. Coating thicknesses were measured using Auger electron sputter depth profiling.The passivations were applied with a thickness recommended by the supplier and thus showed large variation. The thickness of the chromium free passivation (Cr-free) is approximately 75 nm. The coating contains the active ions; H₃O⁺, Ti⁴⁺, Mn²⁺, Zn²⁺, PO₄³⁻. The passivation based on trivalent chromium (Cr-III) is approximately 30 nm thick and contains the active ions; H₃O⁺ Cr³⁺, PO₄³⁻, F. The chromate based passivation (Cr-VI) is approximately 5 nm thick and contains the active ions Cr⁶⁺/Cr³⁺, F⁻.The Cr-free and the Cr-III passivations showed similar white rust protection in the corrosion tests. The corrosion resistance was good although it did not fully reach the level of the Cr-VI passivation. The results from the tests of the painted panels showed that the powder paint worked well on all three passivations. The solvent born paint worked best on the passivation based on trivalent chromium. The water born paint showed poor resistance to blistering in the Cleveland humidity test for all three passivations. In this test the passivation with hexavalent chromium showed slightly better results than the chromate free passivations.     

Determination of instantaneous corrosion rates and runoff rates of copper from naturally patinated copper during continuous rain events

Instantaneous corrosion rates of naturally patinated copper of varying age (16 months, 138 and 145 years) have been determined during continuous rain events in the laboratory with electrochemical impedance spectroscopy using a two-electrode cell. The two-electrode cell was found to yield the same information in bulk rainwater as a conventional three-electrode cell.Relatively constant corrosion rates, between 0.2 and 0.6 μm/y, were determined for samples having a two-layer structure with an inner brownish layer of cuprous oxide and an outer greenish layer of basic copper salts (138, 145 years). Samples with cuprous oxide as the dominating phase of the patina (16 months) showed higher and somewhat increasing corrosion rates during a rain event (from 0.6 to 1.2 μm/y). During a continuous rain event, corrosion rates were found to be approximately 10 (brownish patina) and 25 times (greenish patina) lower than corresponding instantaneous runoff rates. The first flush phenomena of the runoff process, with an increased concentration during first flush and a relatively constant concentration during steady state, was indirectly seen as an increase in solution conductivity during the first rain volume followed by relative constant value. The contribution of the concentration in the first flush to the total annual runoff rate was significant for panels having a greenish layer (138, 145 years) whereas it was negligible for panels having a brownish layer (16 months).     

Atmospheric corrosion evaluation of galvanised steel by thin layer activation

The release of certain metals, such as zinc, from outdoor constructions due to atmospheric corrosion is of some concern. For risk assessments the evaluation of the amount of released metal is of importance. Various methods can be used to study the release of metals. These include those using radiotracers, such as thin layer activation (TLA). To verify the reliability of TLA with respect to conventional techniques in the evaluation of atmospheric corrosion, galvanised steel was exposed to a mild marine environment. The amount of zinc in the corrosion products, released through artificial leaching, at different time intervals was evaluated by TLA and atomic absorption spectroscopy (AAS). A good correlation between the results was found indicating the feasibility of TLA for these release studies.     

An assessment of climate change effects on atmospheric corrosion rates of steel structures

Abstract

This paper presents a discussion on assessing the potential impacts of climate change on the atmospheric corrosion rates of exposed steel structures. The effects on atmospheric corrosion due to changes in the environmental temperature, carbon dioxide, relative humidity, wind, rainfall and pollution are considered. The limitations and complexities of these assessments are discussed. To demonstrate the use and limitations of this science to evaluate effects related to climate change, a model developed in Australia to predict corrosion is combined with climate change models to project the change in the corrosion rates of steel components and protective zinc coatings in constructions. The method is applied to constructions located along the coastal areas of two Australian cities: Melbourne and Brisbane. These assessments are made using the A1FI scenario, the highest emission scenario defined by the Intergovernmental Panel on Climate Change, applied to nine general circulation models. The projected changes in corrosion rates were found to be an increase of ～14% for both zinc and steel in Brisbane and a decrease of ～14% for steel and 9% for zinc in Melbourne. It was also found that the uncertainties associated with the climate change models were small compared to those involved in modelling corrosion for engineering purposes.     

Atmospheric corrosion in subtropical areas: Statistic study of the corrosion of zinc plates exposed to several atmospheres in the province of Santa Cruz de Tenerife (Canary Islands, Spain)

A semi-empiric equation has been proposed in order to predict corrosion rate values of zinc samples previously exposed to subtropical atmospheres. Samples were exposed during four years to the action of different atmospheres in 35 test sites distributed in the western islands of the Canarian Archipielago (Spain) located in front of the African continent. Corrosion rate has been obtained by lost of weight. Time of wetness (T_w) and deposition rates of atmospheric pollutants (SO₂ and Cl⁻) have also been determined. Considering the meteorological data (SO₂ and Cl⁻ deposition rates) and corrosion rate values corresponding to three years of exposure, a semi-empiric equation that predicts, with good agreement, the corrosion rate of zinc exposed to this subtropical atmosphere, has been proposed. The validity of the equation has been tested comparing the equation predictions with experimental data corresponding to samples with four years of exposure. Both experimental and predicted data showed a good agreement.     

Influence of tensile stress on corrosion behaviour of high‐strength galvanized steel bridge wires in simulated acid rain

Corrosion behaviour of the high‐strength galvanized steel wires under tensile stress was researched by electrochemical polarization and salt spray test (SST) using simulated acid rain as electrolyte. Electrochemical polarization and SST results showed corrosion rate rose significantly with increasing tensile stress; white grains were observed by SEM after polarization, while cellular and dendritic crystals appeared on the rust layer after SST. XRD and TG‐DTA results revealed (Zn(OH)₂)₃ · ZnSO₄ · 5H₂O was the main corrosion product, and traces of Fe₂(SO₄)₂O · 7H₂O, Fe₂(SO₄)₃, Fe₂O₃ · H₂O were also detected. A three‐stage corrosion process for the galvanized steel wires during SST was proposed.     

Pitting corrosion of aluminium in the tropical climate of Mauritius

Aluminium is a commonly used metal in Mauritius. However, the tropical Mauritian atmosphere is quite corrosive with the highest measured corrosivity, according to ISO 9223, falling in the category C4. Hence, this study was performed to investigate the atmospheric corrosion behaviour of aluminium AA1050 in the island. The aluminium samples were exposed outdoors at three sites, according to ISO 8565. Upon removal after specific time intervals over a 1½ years period, pit depth, pit diameter and pit density of the samples were determined. The maximum pit depth varied with the time of exposure, according to the power law function. Pit diameter increased linearly with the time of exposure. Chart rating, according to ISO 11463, was also performed. It was found that the corrosivity of the Mauritian atmosphere falls in categories A4-B1-C1 and A5-B1-C1. Application of extreme value statistics showed that Gumbel type 1 distribution fits maximum pit depth data at the three sites.     

Effect of Caand Mg on corrosion and scaling of galvanized steel pipe in simulated geothermal water

In this paper, the effects of scaling ions (Ca²⁺ and Mg²⁺) on corrosion and scaling processes of galvanized steel pipe in geothermal water are presented. Spherical corrosion products and needle-shaped scale coexisted on the pipe surface. The concentration of Zn²⁺ and OH⁻ affected the nuclei formation of scale. The corrosion products and scale were identified as Zn(OH)₂, ZnO, CaCO₃ and MgCO₃, respectively. When scale formed on the galvanized steel pipe, the corrosion rate slowed down and the pitting region became smaller.     

Indoor atmospheric corrosion of copper and steel under heat trap conditions in Cuban tropical climate

Abstract

Corrosion behaviour of copper and steel under heat trap conditions in Cuban tropical climate is reported. Temperature and humidity reach higher values than those reported for traditional outdoor and indoor conditions. Annual calculated time of wetness is in the range corresponding to outdoor or ventilated sheds. This behaviour is not reported for other indoor conditions. Sulphur compounds deposition rate is higher than chloride deposition rate at all corrosion stations. Main corrosion products formed on steel and copper are goethite and brochantite respectively. No significant differences in the statistical influence of exposure time and time of wetness on atmospheric corrosion process of copper and steel under heat trap conditions are determined.     

Development of a Molybdate-Phosphate-Silane-Silicate (MPSS) coating process for electrogalvanized steel

Electrogalvanized steel samples have been treated in a solution containing molybdate, phosphate and silicate and in the same solution containing silane and nitric acid as additives. The corrosion resistance of the treated samples has been compared to that of bare samples after exposure to 0.5 N NaCl for 24 h using polarization curves and EIS data. The chemical composition of the coating layers has been determined using different surface analysis techniques. The samples have also been exposed to the salt spray test. The results of these tests suggest that the MPSS treatment is a promising candidate for replacing conventional chromate treatments.     

Atmospheric corrosion in subtropical areas: XRD and electrochemical study of zinc atmospheric corrosion products in the province of Santa Cruz de Tenerife (Canary Islands, Spain)

In the present paper, zinc sheets have been exposed for 4 years to the action of different atmospheres in 35 test sites located in the province of Santa Cruz de Tenerife, Canary Islands, Spain. Corrosion products formed on the surface of the samples have been identified by means of X-ray diffraction (XRD) for the first and second year of exposure. Zincite, hydrozincite, simonkolleite, zinc chlorohydroxysulphate, zinc oxysulphate and zinc hydroxysulphate have been identified in the test sheets. Preliminary results of an electrochemical study of the breakdown potential of zinc samples are also presented in order to test the protective effect of the film formed on the surface of the samples. It was found that the protective effect of this film increases linearly with exposure time.     

Microbial corrosion of galvanized steel by a freshwater strain of sulphate reducing bacteria (Desulfovibrio sp.)

Microbially influenced corrosion of galvanized steel was investigated exposing coupons (2.0 × 2.0 cm) to culture of sulphate reducing bacteria strain Desulfovibrio sp. The coupons were exposed to the SRB culture for 4, 8, 24, 72, 96, 168, 360 and 744 h along with a control set in uninoculated medium. The results from the present study suggest that SRB were responsible for the corrosion of the galvanized steel. The corrosion rate of galvanized steel was not be correlated with the number of sessile SRB cells. Also Desulfovibrio sp. had an ability to produce EPS (Extracellular Polimeric Substances) and biodegrade the carbohydrate, that is the predominant component of the EPS produced by them. When Desulfovibrio sp. cells were in logarithmic phase, the concentration of dissolved zinc in the bulk solution was very toxic for Desulfovibrio desulfuricans.