Investigating corrosion processes in the micrometric range: A SVET study of the galvanic corrosion of zinc coupled with iron

The galvanic corrosion of an iron/zinc pair immersed in aqueous 0.1 M Na₂SO₄ solution has been investigated by using the scanning vibrating electrode technique (SVET). In this way, investigations in the micrometer range of the progress of the electrochemical reactions involved in galvanic process were performed. The anodic oxidation process is observed to be initiated on the zinc sample in a localized manner, whereas the cathodic reaction involving the electroreduction of dissolved oxygen is homogeneously distributed over the iron sample. This later process is the rate determining step in the overall corrosion process, as demonstrated by the changes in the ionic and galvanic currents measured in the system when the area of the iron specimen is varied relative to that of zinc. The occurrence of coupled chemical reactions in the solution phase involving the products of the corrosion reactions could also be deduced from the integration of the ionic currents measured for each half-reaction during a SVET scan. Thus, the corrosion processes involved in the galvanic coupling of iron and zinc have been further understood by using this microelectrochemical technique appropriately, helping to better interpret large scale measurements.     

Atmospheric corrosion resistance of MnCuP weathering steel in simulated environments

In this work, atmospheric corrosion resistance of low cost MnCuP weathering steel in simulated coastal, industrial, and coastal–industrial atmospheric environments was investigated by wet/dry cyclic acceleration corrosion tests. The results indicate that MnCuP weathering steel exhibits high corrosion resistance in the three atmospheres. Besides, the alloying effect of Mn, Cu, and P elements on the anti-corrosion mechanism of MnCuP weathering steel was discussed by techniques of X-ray photoelectron spectroscopy, potential–pH diagram, and electron probe microanalysis.     

Atmospheric corrosion of Galfan coatings on steel in chloride-rich environments

Galfan coatings on steel in laboratory exposures with predeposited NaCl and cyclic wet/dry conditions exhibit nearly the same corrosion products as after 5 years of marine exposure. A general scenario for corrosion product evolution on Galfan in chloride-rich atmospheres is proposed. It includes the initial formation of ZnO, ZnAl₂O₄ and Al₂O₃ and subsequent formation of Zn₆Al₂(OH)₁₆CO₃⋅4H₂O, and Zn₂Al(OH)₆Cl⋅2H₂O and/or Zn₅Cl₂(OH)₈⋅H₂O. An important phase is Zn₆Al₂(OH)₁₆CO₃⋅4H₂O, which largely governs the reduced long-term zinc runoff from Galfan. A clear influence of microstructure could be observed on corrosion initiation in the slightly zinc-richer η-Zn phase adjacent to the β-Al phase.     

Effect of molybdate post-sealing on the corrosion resistance of zinc phosphate coatings on hot-dip galvanized steel

The technique of post-sealing the phosphated hot-dip galvanized (HDG) steel with molybdate solution was addressed. The composition and corrosion resistance of the improved phosphate coatings were investigated by SEM, EDS, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements, and neutral salt spray (NSS) test. The results showed that molybdate films were formed in the pores of phosphate coatings, and the compact and complete composite coatings composed of phosphate coatings and molybdate films were formed on the zinc surface, resulting in that both the anodic and cathodic processes of zinc corrosion were inhibited remarkably; the corrosion protection efficiency values were increased; and the electrochemical impedance values were enhanced at least one order of magnitude. The low frequency impedance values for the composite coatings were increased at the initial stages of immersion in 5% sodium chloride solution, indicating the self-repairing activity of the composite coatings.     

Microbial corrosion of galvanized steel in a simulated recirculating cooling tower system

In this study, mixed species biofilm formation including sulphate reducing bacteria (SRB) on the galvanized steel surfaces and also microbiologically influenced corrosion (MIC) of galvanized steel were observed in a model recirculating cooling water system during 10 months. A biofilm which had a heterogeneous structure formed on galvanized steel coupons. The results suggested that galvanized steel was corroded by microorganisms as well as SRB in the biofilm. Extracellular carbohydrate was degraded and quantities of carbohydrate were positively correlated with the weight loss. The concentrations of zinc in the biofilm showed significant correlations with weight loss, carbohydrate amount and SRB count.     

Effect of cations on corrosion of zinc and carbon steel covered with chloride deposits under atmospheric conditions

The effect of sodium, calcium, and magnesium chlorides deposited on zinc and carbon steel surfaces was studied under atmospheric conditions. The cations strongly affected the corrosion rate of zinc, whereas they had a significantly lower impact on the corrosion of carbon steel. The corrosivity of cations of chloride salts for zinc increased in order of Mg²⁺ < Ca²⁺ < Na⁺. The higher corrosion resistance of zinc treated with calcium and magnesium chlorides was connected to prevention of formation of hydrozincite during zinc exposure in wet air. It was observed that zinc weight loss and the carbonate to simonkolleite ratio in corrosion products were correlating. The principal protective effect of bivalent cations can be seen in the decrease of pH of the surface electrolyte, which was caused by hydrolysis of such cations and subsequent formation of simonkolleite that blocked the cathodic sites.     

An electrochemical study of the delamination of polymer coatings on galvanized steel

The relationship between the rate of polymer delamination and the intensity of either anodic or cathodic current under the paint has been investigated for the zinc/surface treatment/polymer system by using a special electrochemical cell. Three types of surface treatment were investigated: simple alkaline degreasing, trication phosphating, and a chromate free conversion coating. Significant differences were observed for the three substrates. The alkaline resistance of the conversion coatings was determined using an ICP atomic emission spectroelectrochemical method. The results are interpreted in terms of the differing chemical stability of the conversion layers towards hydroxide generated by oxygen reduction.     

Understanding corrosion via corrosion product characterization: II. Role of alloying elements in improving the corrosion resistance of Zn–Al–Mg coatings on steel

Corrosion products are identified on Zn, ZnMg and ZnAlMg coatings in cyclic corrosion tests with NaCl or Na₂SO₄ containing atmospheres. For Mg-containing alloys the improved corrosion resistance is achieved by stabilization of protective simonkolleite and zinc hydroxysulfate. At later stages, the formation of layered double hydroxides (LDH) is observed for ZnAlMg. According to thermodynamic modeling, Mg²⁺ ions bind the excess of carbonate or sulfate anions preventing the formation of soluble or less-protective products. A preferential dissolution of Zn and Mg at initial stages of corrosion is confirmed by in situ dissolution measurement. The physicochemical properties of different corrosion products are compared.     

Degradation mechanism of galvanized steel in wet-dry cyclic environment containing chloride ions

The wet-dry cyclic test of a galvanized steel (GI) and pure zinc (ZN), which simulates marine atmospheric environment, has been conducted to clarify the degradation mechanism of galvanized steel. The samples were exposed to alternate conditions of 1 h-immersion in a 0.05 M NaCl solution and 7 h-drying at 25 °C and 60%RH, and the corrosion was monitored for 10 days (30 cycles) using a two-electrode type probe. Simultaneously, the corrosion potential was measured every three cycles only during the immersed conditions. The reciprocal of polarization resistance R_p⁻¹ was taken as an index of the corrosion rate. Several sample plates of GI and ZN were exposed, together with the monitoring probes. They were removed from the test chamber at the end of 1st, 3rd, 9th, 18th, and 30th cycles of exposure and were analyzed for the corrosion products with XRD and laser Raman spectroscopy. Further, their cross sections were analyzed with FESEM-EDS. The FESEM photographs and elemental analysis of cross sections confirmed that the R_p⁻¹ value commences to decrease when the corrosion front reaches Zn-Fe alloy layers (boundary layers of zinc coating and steel substrate) due to localized nature of attack. A schematic model of degradation mechanism and the role of galvanic protection have been discussed.     

Use of SVET and SECM to study the galvanic corrosion of an iron-zinc cell

The work makes use of the scanning vibrating electrode technique (SVET) and the scanning electrochemical microscope (SECM) to investigate microscopic aspects of the electrochemical reactions that occur in an iron-zinc galvanic couple immersed in aqueous sodium chloride solution. Detection of the corrosion processes was made by sensing the phenomena occurring in solution. The SVET provided information on the distribution of ionic currents arising from the metal surface, whereas the SECM measured the concentration of chemical species relevant to the corrosion processes. The two techniques had comparable sensitivity for the corrosion of iron but significant differences were observed concerning the detection of corrosion of zinc.     

Prohesion and outdoors tests on corrosion products developed over painted galvanized steel sheets with and without Cr(VI) species

Galvanized steel sheets pre-treated with a new product and then painted with a polyester topcoat without primer were submitted to Prohesion G-85 test (PT) and to outdoor marine exposure test (OT). The new product that replaces standard inorganic chromium pre-treatment + primer consists in a water based resin which applied directly to the properly cleaned metal substrate is then dried in place. This scheme sets aside Cr(VI) compounds which cause severe damages on human health.Goethite, lepidocrocite, magnetite, akaganeite and silicates were found in OT samples coinciding with the usual corrosion products obtained for conventional painting schemes (with Cr(VI)).Surprisingly in PT samples greigite was detected, showing that the new painting scheme is susceptible to microbiological corrosion. Goethite, lepidocrocite, pyrite, magnetite and akaganeite were also found.This study allows the conclusion that in the PT the corrosion mechanism is different from that in the OT for the analyzed samples and should not be used to predict the performance of this kind of outdoor exposed materials.     

Atmospheric corrosion of field-exposed magnesium alloy AZ91D

The magnesium alloy AZ91D was exposed in three different types of atmospheric environment, viz. urban, rural and marine exposure sites. Corrosion rates, corrosion products formed, and the influence of the microstructure on the corrosion behaviour of the alloy were investigated. The corrosion rate of AZ91D exposed in the marine environment was 4.2 μm/year, and in the rural and urban environments 2.2 and 1.8 μm/year, respectively. The main corrosion product found was magnesium carbonate hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), which was formed at all three exposure sites. The corrosion attack started in the -phase in larger grains at the boundary between the -phase and the eutectic -/β-phase. Microgalvanic elements were formed with the eutectic -/β-Mg phase as cathodic site and the -Mg grains as anodes. The Al–Mn particles played a minor roll in the initiation process, even though these particles are the most noble in the microstructure and thus the driving force for a corrosion attack around these particles could be expected to be high. A close resemblance was observed between the corrosion mechanisms operating under the field-exposure conditions described here and the mechanisms operating under the previously reported laboratory conditions.     

Study of localized corrosion of 304 stainless steel under chloride solution droplets using the wire beam electrode

Localized corrosion of 304 stainless steel under droplets of 1 M sodium chloride solution was investigated by the wire beam electrode (WBE) method. It was found that the current distributions were heterogeneous with isolated anodic current peaks mostly located near the edge of the droplet. During the corrosion process, the stainless steel WBE exhibited the stochastic characteristics with the disappearance of some anodic sites. In addition, stainless steel suffered more serious localized corrosion with the increase of the droplet size. The increase of the cathodic area and the three-phase boundary (TPB) length was believed to be the reason.     

Effect of cobalt on the corrosion resistance of low alloy steel in sulfuric acid solution

This study examines the effect of cobalt addition on the corrosion resistance of low alloy steel in a 10 wt.% sulfuric acid solution at room temperature. All specimens show passive behaviors over the potential range, 0.5–1.55 V_SCE, and the passive current density decreased with increasing Co addition. The Co-containing steels also show higher polarization resistance and lower corrosion rate. The beneficial effect of Co is attributed to the formation of a uniform and continuous rust layer due to an interaction between Co and other elements such as Fe, O and S.     

The effect of synthetic zinc corrosion products on corrosion of electrogalvanized steel. II. Zinc reactivity and galvanic coupling zinc/steel in presence of zinc corrosion products

The reactivity of zinc under synthetic zinc patinas and the galvanic coupling in steel/patina/Zn are studied. Zn₅(OH)₆(CO₃)₂ and Na₂Zn₃(CO₃)₄⋅3H₂O inhibit zinc anodic dissolution in NaCl, while Zn₅(OH)₈Cl₂ H₂O and Zn₄(OH)₆SO₄ nH₂O do not. The galvanic current in steel/patina/NaCl/Zn is smaller as compared to steel/NaCl/Zn. The inhibiting effect decreases with time for Na₂Zn₃(CO₃)₄⋅3H₂O or Zn₄(OH)₆SO₄ nH₂O due to the transformation into Zn(OH)₂. In NaHCO₃, the polarity between zinc and steel can reverse. The effect of confinement on the cathodic current is stronger than the initial effect of patina which is explained by the instability of the patinas under rapid pH-increase.     

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.     

Application of automated electrical resistance sensors for measurement of corrosion rate of copper,bronze and iron in model indoor atmospheres containing short-chain volatile carboxylic acids

The corrosion rate of copper and bronze Cu-8 wt.%Sn increased rapidly when the concentration of formic or acetic acid in air reached about 300 ppb at 80% relative humidity (RH) and a temperature of 20 °C. It decreased slowly during the several days after pollutant removal due to the slow rate of pollutant desorption from the metal surfaces. Corrosion of these metals was barely affected by the acids at RH up to 60%. For iron, the critical concentration of formic acid in air which led to surface activation at 80% RH was between 1000 and 1590 ppb.     

Effect of prior corrosion on short crack behavior in 2024-T3 aluminum alloy

Two thicknesses of dogbone shaped 2024-T3 aluminum alloy specimens were notched and corroded prior to constant amplitude fatigue loading. The purpose of the subject research was to examine and characterize the effects of various levels of prior corrosion on the growth rate of short fatigue cracks. The specimens were notched and exposed to a corrosive environment per one of three defined protocols prior to experimentation. The notch was manually introduced at one edge of the test section of the specimen, which was later corroded to create a more natural site for crack origination. Fatigue crack nucleation was monitored and subsequent crack growth recorded, with results presented in the form of da/dN vs. ΔK curves.     

Effect of carbon dioxide on the atmospheric corrosion of Zn–Mg–Al coated steel

The atmospheric corrosion of line hot dip ZnMgAl coating was investigated at low and ambient concentration of CO₂ as a function surface chloride concentration and temperature and compared to conventional hot dip galvanised (GI) and Galfan coatings. The corrosion of zinc coatings was enhanced in low CO₂ conditions and ZnMgAl material was more affected than GI, and in the range of the Galfan coating. An obvious pH effect was underlined in low CO₂ conditions. Layered double hydroxide (LDH) and simonkolleite were mainly formed on ZnMgAl coating in the absence of CO₂ while hydroxycarbonate and simonkolleite were dominating in ambient air.     

The role of rusts in corrosion and corrosion protection of iron and steel

The processes of atmospheric corrosion of iron and steel and the properties of corrosion products (rusts) are modeled based on a quantitative evaluation of the chemical reactions pertaining to corrosion to elucidate the conditions with which corrosion-protective rust films form. Based on the model, it is suggested that in the initial stage of corrosion, in the rusts, the pH of the aquatic system is maintained at 9.31 owing to an equilibrium with iron(II) hydroxide and the rate of air-oxidation at this pH is very fast, and that dense, self-repairing rust films form, protecting the underlying iron and steel. However, after corrosion stops, the rust film deteriorates due to the dissolution and shrinkage by aging, and the deteriorated rust film separates the anode and cathode reaction products (Fe²⁺ and OH⁻ ions) to cause crevice corrosion. The air-oxidation of iron(II) in anode channels without the presence of OH⁻ ions results in strongly acidic solutions (pH 1.41), causing acid-corrosion. It is proposed that good catalysts (e.g. copper(II) and phosphate ions) accelerate the air-oxidation at low pH, delaying the crevice- and acid-corrosion stages. Further, it is argued that iron compounds with negative charges due to the non-stoichiometric proportions of the lattice oxide ions and metal ions (solid oxoanions of iron) exhibit stable cation-selective permeability even with a drop in pH. Rust films including such compounds would stop the passage of aggressive anions and act to protect iron and steel.