Initial Corrosion Rate Research Articles

Hydrogen Sulphide Corrosion of Carbon and Stainless Steel Alloys Immersed in Mixtures of Renewable Fuel Sources and Tested Under Co-processing Conditions

Abstract In accordance with modern regulations and directives, the use of renewable biomass materials as precursors for the production of fuels for transportation purposes is to be strictly followed. Even though, there are problems related to processing, storage and handling in wide range of subsequent uses, since there must be a limit to the ratio of biofuels mixed with mineral raw materials. As a key factor with regards to these biomass sources pose a great risk of causing multiple forms of corrosion both to metallic and non-metallic structural materials. To assess the degree of corrosion risk to a variety of engineering alloys like low-carbon and stainless steels widely used as structural metals, this work is dedicated to investigating corrosion rates of economically reasonable engineering steel alloys in mixtures of raw gas oil and renewable biomass fuel sources under typical co-processing conditions. To model a desulphurising refining process, corrosion tests were carried out with raw mineral gasoline and its mixture with used cooking oil and animal waste lard in relative quantities of 10% (g/g). Co-processing was simulated by batch-reactor laboratory experiments. Experiments were performed at temperatures between 200 and 300ºC and a pressure in the gas phase of 90 bar containing 2% (m3/m3) hydrogen sulphide. The time span of individual tests were varied between 1 and 21 days so that we can conclude about changes in the reaction rates against time exposure of and extrapolate for longer periods of exposure. Initial and integral corrosion rates were defined by a weight loss method on standard size of coupons of all sorts of steel alloys. Corrosion rates of carbon steels indicated a linear increase with temperature and little variation with composition of the biomass fuel sources. Apparent activation energies over the first 24-hour period remained moderate, varying between 35.5 and 50.3 kJ mol−1. Scales developed on carbon steels at higher temperatures were less susceptible to spall and detach. Nonetheless, moderate deceleration of corrosion rates as a function of time are due to the less coherent, frequently spalling and low compactness, higher porosity of the scales evolved at lower and higher temperatures, respectively. On the surface of high alloy steels, sulphide scales of an enhanced barrier nature formed during the induction periods and the layer formation mechanism was found to be assisted by the increasing temperature as initial reaction rates considerably decreased over time. Nevertheless, corrosion-related sulphide conversion of metals and mass loss of the high alloys are strongly affected by the composition of the biomass fuel sources especially animal waste lard. Thermal activation in the first 24 hours decreased from 68.9 to 35.2 kJ mol−1. A greater degree of failure to develop protective sulphide scales was experienced by changing to composition of the biomass fuel sources than the impact of thermal activation between a narrow temperature range at around 100ºC. In accordance with the literature, high free fatty acid contents lead to high corrosion rates accounted for direct corrosion of high alloy steels and assisted solubilisation of corrosion products. In addition, the pronounced acceleration of sulphide corrosion is connected to the diminishing inhibition effect of the sulphide scales.

Extremely High-Rate, Uniform Dissolution of Alloy 22 in Anhydrous Organic Solutions at Room Temperature

During routine pharmaceutical development and scale-up work, severe corrosion of a Hastelloy Alloy C-22 (Alloy 22) filter dryer was observed after single, short (several hour) contact with the product slurry at room temperature. Initial investigations showed that the presence of both 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and HCl was sufficient in an acetonitrile solution to cause rapid corrosion of Alloy 22. More detailed mass loss studies showed initial corrosion rates exceeding 25 mm/y that then decreased over several hours to steady state rates of 3 mm/y to 5 mm/y. The corrosion was highly uniform. Electrochemical measurements demonstrated that although Alloy 22 is spontaneously passive in acetonitrile solution, the presence of HCl leads to the development of a transpassive region. DDQ is a sufficiently strong oxidizer, particularly in acidic solutions, to polarize the Alloy 22 well into the transpassive region, leading to the observed high-corrosion rates.

Corrosion Behavior of Calcium Phosphate-Coated Biomedical Magnesium Alloy Under in Vitro and In Vivo Environments

Magnesium alloys attract attention as a biodegradable/bioabsorbable medical device because of the biodegradability and good mechanical integrity. Corrosion rate of the magnesium alloys should be controlled depending on the implantation part of body as well as the biocompatibility should be improved. Hydroxyapatite (HAp) and octacalcium phosphate (OCP) coatings were developed to control the corrosion rate and to improve the hard tissue compatibility. Corrosion behavior of HAp- and OCP-coated Mg-3mass% Al-1mass% Zn (AZ31) was examined in cell culture medium (in vitro) or in subcutaneous tissue of mouse (in vivo). Effect of medium-replacement and addition of gelatin in medium on the corrosion behavior was also examined. Chemically polished AZ31 was used as a reference. Concentration of Mg ions in medium was quantified. Apparent corrosion initiation of substrate AZ31 was delayed for ca. 1 week with HAp and OCP coatings in medium while Mg ion release from as-polished AZ31 initiated from the beginning. Filiform corrosion took place, but visible growth of filiform corrosion stopped within several days. Afterwards, Mg ion release continued for 1 year with almost a constant corrosion rate. The corrosion rate of AZ31 alloy in medium was reduced by more than 50% with HAp and OCP coatings. HAp coating showed superior corrosion protection effect than OCP coating. After 1 year of immersion, cross section of the immersed specimens showed that uniform corrosion took place underneath the coatings. The wall of filiform corrosion pits was covered with thick corrosion products layer. These results indicate that corrosion continued for long time under the coating and corrosion products layer in medium. The periodical medium replacement during immersion test did not significantly change the initial corrosion rate for 4 weeks. However, the medium-replacement rather reduced the corrosion rate of HAp-coated AZ31 comparing to the result without medium-replacement. Deposition of calcium phosphate from medium was enhanced with the medium-replacement, leading to the improvement of corrosion protection effect of HAp coating. As-polished AZ31 caused the formation of gas cavity in subcutaneous tissue of mouse due to H2 gas generated in corrosion reaction. Such a gas cavity formation was not observed around the implanted HAp- and OCP-coated specimens, indicating that corrosion of substrate AZ31 was also reduced by HAp and OCP coating in subcutaneous tissue. After 16 weeks of implantation, the specimens were retrieved from the mouse and surface and cross section was observed using SEM. Macro- and micro-pits were observed on the surface and underneath the coatings. Unlike in medium, corrosion pits appeared to grow in depth direction rather than in lateral direction in subcutaneous tissue. Because body fluid circulation in the subcutaneous tissue is limited, to simulate the soft tissue environment, gelatin was added in medium and HAp-coated AZ31 was immersed for 4 weeks. Then, the similar corrosion morphology with macro- and micro-pits was obtained in the gelatin-added medium. These results indicate that corrosion of HAp- and OCP-coated AZ31 is localized in diffusion limited environment. The developed HAp and OCP coatings reduced the corrosion rate of Mg alloy in medium and in soft tissue. The corrosion morphology, especially shape of local corrosion, depended on the corrosion environment. The decrease of mass transfer in the environment caused the localization of corrosion of Mg alloy.

Cell Adhesion on Surface-Functionalized Magnesium.

The biocompatibility of commercially pure magnesium-based (cp Mg) biodegradable implants is compromised of strong hydrogen evolution and surface alkalization due to high initial corrosion rates of cp Mg in the physiological environment. To mitigate this problem, the addition of corrosion-retarding alloying elements or coating of implant surfaces has been suggested. In the following work, we explored the effect of organic coatings on long-term cell growth. cp Mg was coated with aminopropyltriehtoxysilane + vitamin C (AV), carbonyldiimidazole (CDI), or stearic acid (SA). All three coatings have been previously suggested to reduce initial corrosion and to enhance protein adsorption and hence cell adhesion on magnesium surfaces. Endothelial cells (DH1+/+) and osteosarcoma cells (MG63) were cultured on coated samples for up to 20 days. To quantify Mg corrosion, electrochemical impedance spectroscopy (EIS) was measured after 1, 3, and 5 days of cell culture. We also investigated the speed of initial cell spreading after seeding using fluorescently labeled fibroblasts (NIH/3T3). Hydrogen evolution after contact with cell culture medium was markedly decreased on AV- and SA-coated Mg compared to uncoated Mg. These coatings also showed improved cell adhesion and spreading after 24 h of culture comparable to tissue-treated plastic surfaces. On AV-coated cp Mg, a confluent layer of endothelial cells formed after 5 days and remained intact for up to 20 days. Together, these data demonstrate that surface coating with AV is a viable strategy for improving long-term biocompatibility of cp Mg-based implants. EIS measurements confirmed that the presence of a confluent cell layer increased the corrosion resistance.

Comparative Study of the Corrosion Behavior of Low-Nickel AISI 202 and Conventional AISI 304 Stainless Steels in Citric Acid Using Electrochemical Techniques

Stainless steel (SS) is one of the most commonly used metallic food contact materials. It may be classified based on its microstructure whether ferritic, austenitic, martensitic, duplex or precipitation hardened. Austenitic SS, among mentioned grades, has the largest contribution to market due to its numerous industrial and domestic applications. In this study, the corrosion behavior of AISI 202 SS – a cheaper grade of stainless steel, in three different solution temperatures of citric acid was investigated using different electrochemical techniques such as open-circuit potential (OCP) measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results were compared to that obtained from conventional AISI 304 SS. OCP, polarization and impedance measurements agreed that AISI 202 SS has comparable resistance to that of AISI 304 SS in citric acid at ambient temperature and at 50 °C. At 70 °C, results of OCP measurements suggest that AISI 304 SS exhibited greater performance as indicated by more positive OCP values in the designated solution. EIS results indicate that the two alloys have identical corrosion resistance even at 70 °C as indicated by their comparable polarization resistance (Rp). The corrosion mechanism in both alloys is charge-transfer controlled as indicated by depressed semi-circular appearance of the generated Nyquist plots. The values of corrosion current densities (icorr) extracted from polarization curves indicate that the initial corrosion rates were higher in AISI 304 than AISI 202 SS suggesting that formation of more protective film may have occurred on the former alloy.

Castable cements to prevent corrosion of metals in molten salts

Castable cements on metals form a protective barrier that is able to prevent permeation of molten salts towards metallic surfaces. Silica-based castable cements are capable of protecting containment metallic alloys from the corrosive attack of molten chlorides at temperatures as high as 650°C. Boron nitride (BN) blocking the pores in the cured cement prevents permeation of the molten chloride towards the metal surface. The cements tested are not chemically stable in molten carbonates, because the bonding components dissolved into molten carbonates salt. The corrosion rate is 7.72±0.32mm/year for bare stainless steel 347 in molten eutectic NaCl – 65.58wt% LiCl at 650°C, which is the baseline used for determining how well the cement protects the metallic surfaces from corrosion. In particular the metal fully encapsulated with Aremco 645-N with pores filled with boron nitride immersed in molten eutectic NaCl – 65.58wt% LiCl at 650°C shows a corrosion rate of 9E-04mm/year. The present study gives initial corrosion rates. Long-term tests are required to determine if Aremco 645-N with BN coating on metal has long term chemical stability for blocking salt permeation through coating pores.

The influence of buffer system and biological fluids on the degradation of magnesium.

The influence of frequently used buffer system 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) compared to CO2 /HCO3- on the corrosion of magnesium is investigated. Samples were immersed in simulated body fluid (m-SBF) while monitored by electrochemical impedance spectroscopy (EIS) for up to 30 days. In CO2 /HCO3- the initial corrosion rate was 0.11 mm yr-1 . An inner protective layer of magnesium oxide was formed within the first 30 min exposure and subsequently covered by an outer layer of apatite within 24 h. The corrosion mechanism thereafter is best described as passive pitting with a porosity of ∼10%. Using HEPES as buffer agent increased the corrosion rate to 3.37 mm yr-1 . Cross sectional microscopy show a porous outer corrosion layer allowing rapid diffusion of aggressive ions through the film. Here the EIS results are best described by an active pitting model with an inner layer 5 to 10 times less protective compared to the inner layer formed without HEPES. Further the suitability of human whole blood and plasma as in vitro models for Mg degradation was evaluated. Mg corrosion caused coagulation after 24 h in both biological fluids. The corrosion during the first 24 h is similar to the corrosion in m-SBF with HEPES. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1490-1502, 2017.

Characterization of the influence of Ni content on the corrosion resistance of electrodeposited Zn–Ni alloy coatings

Abstract The electrodeposition of Zn–Ni alloy coatings containing less than 11% Ni was performed in a sulfuric acid bath. We investigated the influence of the Ni content in the electrodeposit on the corrosion behavior of the alloy in detail, focusing mainly on the evolution of internal stress. The initial corrosion rates of specimens before salt spray testing were closely related to the internal stress and the Zn–6 wt% Ni coating showed better corrosion resistance than those of the other investigated coating due to the smallest internal stress. However, it was shown that the amount of residual Ni and a concentration reversal between Zn and Ni at the Zn–Ni/steel interface became more significant as the corrosion proceeded. In addition, closely packed nodular grains with hairline cracks and the formation of electrochemically noble γ-Ni 5 Zn 21 phase were observed at the highest Ni concentration of 10 wt%. And the cross-sectional distributions of alloying elements were more homogeneous with increasing Ni content in the deposit. Electrochemical test results with the resulting specimens with different salt spray testing times were also presented. At the highest Ni concentration, the sacrificial property to steel substrate was maintained with extended testing time, showing minimal corrosion current density. Regarding Zn–Ni alloy coatings for automotive applications, a decrease in corrosion depth with increasing Ni content after long-term exposure to salt spray was observed, indicating improved perforation resistance.

The protective role of hydrozincite during initial corrosion of a Cu40Zn alloy in chloride-containing laboratory atmosphere

Hydrozincite, Zn5(CO3)2(OH)6, was recently found to play a key role in reducing corrosion product flaking on Cu–Zn alloys. A fundamental study was undertaken to explore the underlying mechanisms, in particular why hydrozincite can suppress the interaction between chlorides and the alloy surface. Hydrozincite could be formed by exposure of Cu40Zn to air at 70% relative humidity and 1000ppm of CO2 resulting in a surface of decreased wettability. Its presence reduces the initial spreading ability of NaCl-containing droplets and lowers the overall initial corrosion rate when the alloy is exposed to pre-deposited NaCl and wet/dry cycles.

Down-time corrosion in boilers

Down-time corrosion can occur on boiler surfaces, e.g. furnace walls, superheaters, or economizers that are covered with hygroscopic deposits, when the temperature drops under 100°C. This scenario takes place when a boiler is shut-down for cleaning, maintenance, or other reasons, such as unplanned shut-downs. Initially, the dry deposits will absorb moisture from the surrounding air, potentially creating a corrosive environment. After this, corrosive acids can form in the deposits. In this study modified online-corrosion probes were used in combination with deposits taken from 6different boilers at various locations (for example, from the furnace, the superheater, and the economizer), where the fuels have been waste, demolition wood or biomass. The deposits were ground and dried in an oven at 160°C for several hours before exposed to a moist environment (RH 65%) during online measuring of the corrosion rate and the pitting activity. Four types of alloys were tested: low-alloy ST45.8-steel, 9% Cr ferritic P91-steel, austenitic stainless steel 304L, and Ni-based super Alloy 625. The results for ST45.8 show that in biomass boilers a corrosion rate from negligible values up to 0.7mm/year can be reached within a week, while waste-fired boilers can have rates as high as 1.8mm/year. Furthermore, for some samples from waste-fired boilers show a high pitting activity already after 24h. The tests with the P91-steel show values up to 0.16mm/year, for samples from different locations in the boiler. For 304L and Alloy 625 the down-time corrosion was negligible even after a two-week exposure. The ability to follow the down-time corrosion online has provided data that show that even though thought to be negligible, the risk of down-time corrosion is of significance, especially if the fuel is waste. Also, even if the initial corrosion rate is low, it increases during the first 24h due to the exposure to moist environment, motivating immediate cleaning of the boilers after shut-down, especially on surfaces of lower steel quality.

Effects of Soil Properties to Corrosion of Underground Pipelines: A Review

This review concentrates on corrosion properties that expose to soil environment. Forms of corrosion classified with respect to outward appearance and altered physical properties are uniform attack, galvanic corrosion, erosion corrosion, stress corrosion, crevice corrosion, pitting and inter-granular corrosion. A porous soil may retain moisture for a longer period for optimum aeration and indirectly increase the initial corrosion rate. External corrosion is corrosion attack upon the outside of the pipe soil medium and the most failure mechanisms experienced by buried steel pipelines. Many systems possibly in contact with soil have risk to be corroded such as storage tanks and pipelines.

Corrosion behavior of 907 steel under thin electrolyte layers of artificial seawater

The corrosion behavior of 907 steel under thin electrolyte layer (TEL) has been investigated by means of cathodic polarization curve measurement, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The results show that the cathodic diffusion current density presents the variation trend of initial increase and subsequent decrease with the decrease of TEL thickness, and the maximum deposits at 58 μm. The cotangent-hyperbolic impedance (O) is rationally first introduced to study the diffusion process of the reactants through the corrosion products layer with many permeable holes. The initial corrosion rate of 907 steel under different TEL thickness increases with the decrease of TEL thickness except that of 104 μm, whereas the corrosion rate after long time corrosion can be ranked as 104 μm>402 μm>198 μm>301 μm >bulk solution.

Reduction of initial corrosion rate and improvement of cell adhesion through surface modification of biodegradable Mg alloy

In this study, the surface modification of biodegradable pure Magnesium and Mg-5wt%Ca-1wt%Zn alloy was performed through immersion in HBSS, inorganic salt solution and cell media to reduce initial hydrogen evolution and improve cell adhesion. The formation of different CaP-like coatings from immersion of pure Mg and Mg alloy were observed using Cryo FIB analysis and their performances were measured through cell adhesion, quantification of released Mg ions, and cell cytotoxicity assays. The coating layers displayed significant reduction of initial corrosion rate, and cell adhesion for both pure Mg and Mg alloy appeared to be influenced by the amino acids and proteins in the cell media. In general, Mg alloy showed a denser coating layer with higher Ca contents, resulting in greater reduction of initial corrosion rate and improved cell adhesion, when compared to pure Mg. This is due to saturation of Ca around the corrosion site that provided much favorable environmental condition to produce denser calcium phosphate coating mixture. The result from this study suggests that the surface modification of biodegradable Mg alloy by immersion in alkaline solutions can be utilized to obtain ideal biodegradable orthopedic implant material with reduced initial hydrogen evolution rate and improved cell adhesion.

The Effects of Primary Oxy-Salts on Anodizing Magnesium Alloy AZ91D

Anodization is known to be an effective way to slow down the initial corrosion rate of magnesium (Mg) and its alloys. Here, we investigated the specific use of oxy-salts to improve the corrosion resistance of anodizing coatings. Oxy-salts of silicate, phosphate, and carbonate were added separately to a sodium hydroxide alkaline electrolyte used to anodize Mg alloy AZ91D. The process was investigated in terms of anodizing behavior, the surface properties, and the corrosion behavior of AZ91D. Anodizing AZ91D using the silicate- containing electrolyte generated sparks, and produced a thicker and more corrosion-resistant layer than the other oxy-salts. In the process, MgO and SiO2 formed Mg2SiO4 at high temperatures. Coatings from the phosphate- and carbonate- containing electrolyte anodizations did not contain phosphorus or carbon. We also studied the effects of silicate concentration on the corrosion resistance and properties of the surface.

The corrosion of depleted uranium in terrestrial and marine environments

Depleted Uranium alloyed with titanium is used in armour penetrating munitions that have been fired in a number of conflict zones and testing ranges including the UK ranges at Kirkcudbright and Eskmeals. The study presented here evaluates the corrosion of DU alloy cylinders in soil on these two UK ranges and in the adjacent marine environment of the Solway Firth.The estimated mean initial corrosion rates and times for complete corrosion range from 0.13 to 1.9 g cm−2 y−1 and 2.5–48 years respectively depending on the particular physical and geochemical environment.The marine environment at the experimental site was very turbulent. This may have caused the scouring of corrosion products and given rise to a different geochemical environment from that which could be easily duplicated in laboratory experiments.The rate of mass loss was found to vary through time in one soil environment and this is hypothesised to be due to pitting increasing the surface area, followed by a build up of corrosion products inhibiting further corrosion. This indicates that early time measurements of mass loss or corrosion rate may be poor indicators of late time corrosion behaviour, potentially giving rise to incorrect estimates of time to complete corrosion.The DU alloy placed in apparently the same geochemical environment, for the same period of time, can experience very different amounts of corrosion and mass loss, indicating that even small variations in the corrosion environment can have a significant effect. These effects are more significant than other experimental errors and variations in initial surface area.

Corrosion of carbon steel by bacteria from North Sea offshore seawater injection systems: Laboratory investigation

Influence of sulfidogenic bacteria, from a North Sea seawater injection system, on the corrosion of S235JR carbon steel was studied in a flow bioreactor; operating anaerobically for 100days with either inoculated or filtrated seawater. Deposits formed on steel placed in reactors contained magnesium and calcium minerals plus iron sulfide. The dominant biofilm-forming organism was an anaerobic bacterium, genus Caminicella, known to produce hydrogen sulfide and carbon dioxide. Open Circuit Potentials (OCP) of steel in the reactors was, for nearly the entire test duration, in the range −800<E(OCP)/mV (vs. SCE)<−700. Generally, the overall corrosion rate, expressed as 1/(Rp/Ω), was lower in the inoculated seawater though they varied significantly on both reactors. Initial and final corrosion rates were virtually identical, namely initial 1/(Rp/Ω)=2×10−6±5×10−7 and final 1/(Rp/Ω)=1.1×10−5±2.5×10−6. Measured data, including electrochemical noise transients and statistical parameters (0.05<Localized Index<1; −5<Skewness<−5; Kurtosis>45), suggested pitting on steel samples within the inoculated environment. However, the actual degree of corrosion could neither be directly correlated with the electrochemical data and nor with the steel corrosion in the filtrated seawater environment. Further laboratory tests are thought to clarify the noticed apparent discrepancies.

Effects of carbon dioxide plasma immersion ion implantation on the electrochemical properties of AZ31 magnesium alloy in physiological environment

Plasma immersion ion implantation (PIII) is conducted to improve the intrinsically poor corrosion properties of biodegradable AZ31 magnesium alloy in the physiological environment. Carbon dioxide is implanted into the samples and X-ray photoelectron spectroscopy and scanning electron microscopy are used to characterize the materials. The corrosion properties are systematically studied by potentiodynamic polarization tests in two simulated physiological environments, namely simulated body fluids and cell culture medium. The plasma-implanted materials exhibit a lower initial corrosion rate. Being a gaseous ion PIII technique, conformal ion implantation into an object with a complex shape such as an orthopedic implant can be easily accomplished and CO2 PIII is a potential method to improve the biological properties of magnesium and its alloys in clinical applications.

Surface modification of a biodegradable magnesium alloy with phosphorylcholine (PC) and sulfobetaine (SB) functional macromolecules for reduced thrombogenicity and acute corrosion resistance.

Siloxane functionalized phosphorylcholine (PC) or sulfobetaine (SB) macromolecules (PCSSi or SBSSi) were synthesized to act as surface modifying agents for degradable metallic surfaces to improve acute blood compatibility and slow initial corrosion rates. The macromolecules were synthesized using a thiol-ene radical photopolymerization technique and then utilized to modify magnesium (Mg) alloy (AZ31) surfaces via an anhydrous phase deposition of the silane functional groups. X-ray photoelectron spectroscopy surface analysis results indicated successful surface modification based on increased nitrogen and phosphorus or sulfur composition on the modified surfaces relative to unmodified AZ31. In vitro acute thrombogenicity assessment after ovine blood contact with the PCSSi and SBSSi modified surfaces showed a significant decrease in platelet deposition and bulk phase platelet activation compared with the control alloy surfaces. Potentiodynamic polarization and electrochemical impedance spectroscopy data obtained from electrochemical corrosion testing demonstrated increased corrosion resistance for PCSSi- and SBSSi-modified AZ31 versus unmodified surfaces. The developed coating technique using PCSSi or SBSSi showed promise in acutely reducing both the corrosion and thrombotic processes, which would be attractive for application to blood contacting devices, such as vascular stents, made from degradable Mg alloys.

Time Resolved Corrosion of Electrode Supports in PEM Fuel Cells

The potentiodynamic corrosion rates of two carbon supports are discussed. The rapid degradation of a high surface area support illustrates the effects of electrode breakup and surface oxide accumulation, and how these changes can actually accelerate further corrosion. Power law decay of corrosion rate is observed for a graphitized support during potential cycling. The initial corrosion rate is a strong function of the reducing condition applied before the start of cycling, consistent with literature postulation of a reversible passivating surface oxide.

Evaluation of Corrosion Behaviour of Grey Cast Iron and Low Alloy Steel in Cocoa Liquor and Well Water

Corrosion behaviour of cast iron and low alloy steel in cocoa liquor and well water was investigated. The average weight losses of the specimens were measured using digital weighing balance. The results showed that the weight losses of both cast iron and low alloy steel in both media increases with time. Corrosion rate of cast iron in cocoa liquor increases rapidly with time for up to 336 hours (1000 μm/yr), but in well water the rapid rate of corrosion only lasted up to 187 hours (1160 μm/yr) thereafter it continuously dropping until 264 hours (667 μm/yr) after which it remains constant. Low alloy steel corroded faster in cocoa liquor up to 264 hours (200 μm/yr), whereas the initial rapid corrosion rate only lasted up to 168 hours (180 μm/yr) in well water environment. The results revealed that low alloy steel exhibited better corrosion resistance in both media, with cocoa liquor been more aggressive. Thus, low alloy steel will be a better material for piping and pumping system in cocoa processing industries.