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Austin Watson
Austin Watson

Principles And Prevention Of Corrosion



Magnesium is the lightest structural engineering material and bears high potential to manufacture automotive components, medical implants and energy storage systems. However, the practical use of untreated magnesium alloys is restricted as they are prone to corrosion. An essential prerequisite for the control or prevention of the degradation process is a deeper understanding of the underlying corrosion mechanisms. Prior investigations of the formation of gaseous hydrogen during the corrosion of magnesium indicated that the predominant mechanism for this process follows the Volmer-Heyrovský rather than the previously assumed Volmer-Tafel pathway. However, the energetic and electronic states of both reaction paths as well as the charge state of dissolved magnesium have not been fully unraveled yet. In this study, density functional theory calculations were employed to determine these parameters for the Volmer, Tafel and Heyrovský steps to gain a comprehensive understanding of the major corrosion mechanisms responsible for the degradation of magnesium.




Principles and prevention of corrosion


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For marine applications, mild steel remains the number one metal for constructional purposes by virtue of its relatively low cost, mechanical strength and ease of fabrication. Its main drawback is that it corrodes easily in seawater and unless adequately protected, rapidly loses strength which may result in structural failure. The diagram below demonstrates the cycle of corrosion. From the mining of iron oxide, producing steel, to corrosion.


Repairing coatings offshore can be up to 100 times the cost of the initial coating, and NACE International estimates that the total cost of marine corrosion worldwide is between $50-80 billion every year. Source: Maritime Industry. 2018. Maritime Industry. [ONLINE] Available at: -Central/Industries/Maritime-Industry/.


Preventing corrosion requires elimination or suppression using two principal methods, cathodic protection and coatings. Generally, cathodic protection systems are used in conjunction with coating systems.


When two dissimilar metals are immersed in seawater, the metal with the lowest electrical potential will suffer the greatest corrosion. For example, the corrosion rate of mild steel can be controlled by connecting it to zinc as it will then become the anode and corrode. In this example, the zinc anode is referred to as a sacrificial anode because it is slowly consumed (corrodes) during the protection process.


A vessel hull can be made cathodic by using a direct current source. An impressed current is applied in the opposite direction to cancel out the corrosion current and convert the corroding metal from anode to cathode. In this example, the negative terminal of DC is connected to a pipeline to be protected. The anode is kept in to increase the electrical contact with its surrounding environment.


Highly cross-linked, chemically curing systems are likely to have relatively low permeability characteristics and film thickness can affect it. In general, thicker films delay the passage of oxygen and water to the steel surface. High film thickness (>400 um dft) can therefore offer a high degree of corrosion protection which would best be achieved in multi-coat systems rather than in a single coat.


The most common type of iron corrosion occurs when it is exposed to oxygen and the presence of water, which creates a red iron oxide commonly called rust. Rust can also effect iron alloys such as steel. The rusting of iron can also occur when iron reacts with chloride in an oxygen-deprived environment, while green rust, which is another type of corrosion, can be formed directly from metallic iron or iron hydroxide.


One of the most aggressive forms of corrosion, pitting can be hard to predict, detect or characterise. This localised type of corrosion happens when a local anodic or cathodic point forms a corrosion cell with the surrounding surface. This pitt can create a hole or cavity which typically penetrates the material in a vertical direction down from the surface.


Pitting corrosion can be caused by damage or a break in the oxide film or a protective coating and can also be caused through non-uniformities in the structure of the metal. This dangerous form of corrosion can cause a structure to fail despite a relatively low loss of metal.


This form of corrosion occurs in areas where oxygen is restricted such as under washers or bolt heads. This localised corrosion usually results from a difference in the ion concentration between two areas of metal. The stagnant microenvironment prevents circulation of oxygen, which stops re-passivation and causes a buuild-up of stagnant solution moving the pH balance away from neutral.


The imbalance between the crevice and the rest of the material contributes to the high rates of corrosion. Crevice corrosion can take place ar lower temperatures than pitting corrosion, but can be minimised by proper joint design.


Intergranular corrosion occurs when impuraties are present at the grain boundaries which form during solidification of an alloy. It can also be caused by the enrichment or depletion of an alloying element at the grain boundaries. This type of corrosion occurs along or adjacent to the grains, affecting the mechanical properties of the metal despite the bulk of the material being unaffected.


Stress corrosion cracking refers to the growth of cracks due to a corrosive environment which can lead to the failure of ductile metals when subjected to tensile stress, particularly at high temperatures. This type of corrosion is more common among alloys than with pure metals and is dependant on the specific chemical environment whereby only small concentrations of active chemicals are required for catastrophic cracking.


This form of corrosion occurs when two different metals with physical or electrical contact are immersed in a common electrolyte (such as salt water) or when a metal is exposed to different concentrations of electrolyte. Where two metals are immersed together, known as a galvanic couple the more active metal (the anode) corrodes fast than the more noble metal (the cathode). The galvanic series determines which metals corrode faster, which is useful when using a sacrificial anode to protect a structure from corrosion.


The annual worldwide cost of metalic corrosion is estimated to be over $2 trillion, yet experts believe 25 - 30% could be prevented with proper corrosion protection. Poorly planned construction projects can lead to a corroded structure needing to be replaced, which is a waste of natural resources and contradictory to global concerns over sustainability. In addition corrosion can lead to safety concerns, loss of life, additional indirect costs and damage to reputation.


Stress-corrosion cracking (SCC) is observed in machine components when an optimal combination of stress levels and environmental conditions is present in susceptible material. An interesting case history of intergranular SCC failure is presented in this paper. This failure occurred in wear rings that were shrink-fitted on the impellers of a multistage horizontal feed pump during the project procurement phase. After manufacture and assembly, two pumps were performance-tested in the supplier's test loop for a combined total of approximately 600 h. The test duration lasted for approximately three months. During this duration, the two pump internals were not absolutely dry and were exposed to an indoor environment in the supplier's test plant. After the completion of the tests, the pump internals were inspected, cleaned, dried, and packaged before air freighting to the reactor site. The pumps were in storage at the site for approximately two months. When the pumps were opened for inspection, two wear rings on each of the pumps were cracked. This paper summarizes the results of the studies to evaluate the root cause of the wear ring failures and the corrective actions implemented to avoid similar failures.


Disc specimens of 14 mm diameter and 3 mm height were machined from steel rods and then polished to an 800 grit surface finish. Subsequently, the samples were cleaned with distilled water and acetone. All the corrosion studies were carried out in aerated 3.5% NaCl, 0.5 M Na2SO4 and 0.5 M H2SO4 solutions at room temperature; hereafter referred as marine, industrial and chemical environments, respectively. A standard three-electrode cell consisting of the steel as a working electrode, a saturated calomel reference electrode (SCE) and a graphite counter electrode was used to perform the polarization experiments. A magnetic stirrer was used to stir the solutions. The specimens were fixed in a specimen holder and were exposed to the testing environment in such a way that the exposed surface area of each specimen was 1 cm2. A computer-controlled EG & G Potentiostat (Model 273A) was used with corrosion analysis software supplied by Princeton Applied Research Corporation (PARC), USA to record the potentiodynamic impedance, the cyclic polarization and the isolated electrode potential as a function of time for the steel in different environments. AC impedance plots were recorded using a 5210 lock-in amplifier, a Model 273A potentiostat, Model 398 impedance software and a computer. Subsequently, an SEM was used to determine the nature of corrosion in the various environments.


The values of Ecorr shifted to the more electronegative side in the industrial environment compared with the marine environment, whereas in the chemical environment, Ecorr shifted to the more electropositive side. However, it is not possible to interpret on the basis of Ecorr data. Icorr is lowest in the marine environment, moderate in the industrial environment and a high Icorr is observed in the chemical environment. The value of Icorr is directly proportional to the corrosivity of the environment in which the steel is tested. Therefore, the chemical environment is the most corrosive followed by the industrial and marine environments, which is clearly reflected in the corrosion rates. 041b061a72


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