Corrosion acceleration

Aluminum containers are recommended for many appHcations because of the very hard, corrosion-resistant oxide coating. They are deficient in only one respect once the protective skin has been penetrated, aluminum corrosion accelerates. 

Wastage was caused by crevice corrosion, accelerated by the difference in tube and tube sheet metallurgies. The brass tube, being more noble, was cathodically protected by corrosion of the surrounding mild steel tube sheet. However, the galvanic effect was secondary to the primary cause of failure, namely, crevice corrosion. 

Although it is entirely possible for erosion-corrosion to occur in the absence of entrained particulate, it is common to find erosion-corrosion accelerated by a dilute dispersion of fine particulate matter (sand, silt, gas bubbles) entrained in the fluid. The character of the particulate, and even the fluid itself, substantially influences the effect. Eight major characteristics are influential particle shape, particle size, particle density, particle hardness, particle size distribution, angle of impact, impact velocity, and fluid viscosity. 

Rose, R. M., Schiller, A. L. and Radin, E. L., Corrosion-Accelerated Mechanical Failure of a Vitallium Nail-Plate , Journal of Bone and Joint Surgery, 54A, 854-862 (1972)... 

The ASTM G 53 test also shows, though not so clearly, the effect of the chloride contaminant on the integrity of the paint. The DEF-1053 -26 test however does not seem to be sensitive to the presence of this metallic corrosion accelerator. 

The main non-chloride, non-corrosive accelerating admixtures available on the market are of two types (1) accelerating admixtures which accelerate hydration but do not depress the freezing point of water and (2) accelerating admixtures for use in sub-freezing ambient temperatures which depress the freezing point of water. The former contain salts of formates, nitrates and nitrites and are effective for set acceleration and strength development. However, their effectiveness is dependent on the ambient temperature at the time of placement. 

Following are the tests described in this Spec wet tensile strength, adhesion at low temp, moisture-vapor transmission rate, water-penetration rate corrosion, accelerated aging, dry tensile strength and tearing resistance... 

Protective films on metals also can be destroyed and corrosion accelerated by the impingement of a high-velocity stream of sea water onto the metallic surface. The inlet ends of condenser tubes, are frequently attacked (Figure 3,a). Jet tests have been devised for ranking the susceptibility of metals to such impingement attack. A corrosion cell is formed between the bare surface directly under the jet (anode) and the adjacent filmed surface (cathode). These jet tests give more dependable information, if natural sea water is continuously supplied to the equipment. 

Potassium formate and a few other carboxylic acid salts are one result of the breakdown of the DEA molecule. These salts are benign at low concentrations. However, when at concentrations of 5% or more, they interfere with operations by altering the physical properties of the carbonate solution. Some amine degradation compounds are even considered to be corrosion accelerators. They may solubilize iron, keep it in solution and prevent it from forming the passivation layer on the pipe and equipment. UOP s ACT-1 activator is an amine that contains a more-stable molecule that is more resistant to degradation206. 

The addition of various corrosion inhibitors to the epoxy encapsulant formulation to prevent corrosion was also evaluated (8). This procedure basically adds compounds or complexing agents to neutralize or "getter corrosion accelerating impurities or adds hydro-phobic agents to reduce the moisture permeation of the plastic. 

Metallic corrosion usually produces corrosion precipitates of metal oxides and hydroxides on the surface of corroding metals in aqueous solution. Corrosion precipitate layers thus formed affect the corrosion of underlying metals. The presence of porous precipitate layers of insoluble rusts such as hydrous metal salts and hydroxides will cause either corrosion acceleration or corrosion inhibition [64-66],... 

Importance of Solid Corrosion-Product Formation Corrosion Acceleration Versus Passivation... 

Fretting corrosion occurs where there is friction, generally caused by vibrations, between two metal surfaces. The debris formed by fretting corrosion accelerates the initial damage done by contact welding. Vibrations cause contact welds to break with subsequent surface deterioration. Debris formed acts to accelerate this form of corrosion by serving as an abrasive. Fretting corrosion is especially prevalent in areas where motion between surfaces is not foreseen. 

Hoar Ci J found that in the corrosion inhibition of iron in hydrochloric acid by g-naphthoquinoline, the corrosion potential increases monotonically with increasing inhibitor concentration, while in the case of o-tolylthiourea one observes first a decrease of the corrosion potential followed by an increase at higher inhibitor concentrations. A similar predominant inhibition of the cathodic partial reaction at small inhibitor concentrations is exhibited also by phenylthiourea according to Kaesche. Furthermore, in the series of the thiourea derivatives one often finds corrosion acceleration at small concentrations, as for instance in the case of phenylthiourea at concentrations of 10- moles per liter. This appears to be due to a small cathodic decomposition of thiourea and its derivatives in the course of which hydrogen sulfide is formed. As is well known, hydrogen sulfide tends to accelerate corrosion, in particular the anodic partial reaction of dissolution of iron, which has been demonstrated independently by other authors (17). 

Influence of Hydrodynamic Conditions Inhibitors are often applied under conditions of liquid flow (e.g. in cooling systems or pipelines) and, consequently, the effect of the flow rate on their performance also has to be considered. First, the hydrodynamic conditions determine the transport of all dissolved species in the solution (inhibitor, H" ", OH , anions, corrosion products) and hence their concentration at the surface of the corroding metal. Since both inhibiting and corrosion-accelerating species are affected, the effect of transport on the inhibition efhciency is not easy to estimate and wiU depend strongly on the inhibition mechanism. Second, the liquid flow may exert mechanical forces on the metal surface, which may promote corrosion and counteract... 

Because of the complexity of real systems, inhibitor selection is by far not straightforward and requires testing by laboratory and field studies. Commercial formulations usually contain a combination of several active inhibitors, blended with a specific surfactant/solvent package, which controls the release of the inhibitors into the environment (i.e. the available inhibitor concentration). The protection of multimetal systems requires particular care, since efficient inhibitors for one metal may be ineffective or even corrosion-accelerating for another metal. In general, the performance of formulations depends on the system parameters and has to be evaluated by field tests to ensure protection. 

SCAB (Simulated Corrosion Accelerated outdoor corrosion test. Test specimens... 

Corrosion corrosive subject to corrosion corrosion accelerating corrosion inhibiting corroded material ... 

In the case of IRB, the results of a piece of research concluded that the static or nonstatic regime of the environment could have a profound effect on acceleration or inhibition of corrosion. As may be inferred from this research, if the system is closed so that dilution of ferrous ions is not happening, corrosion may be slowed down otherwise, in an open system where dilution of ferrous ions is possible, corrosion acceleration can be expected. 

However, the most catastrophic cases to have happened are two cases caused by a break of a main feed-water pipe with corrosion accelerated by water flow. These happened in the Trojan power station in 1985 (368 mm pipe) and in the Surry 2 power station in 1986 (460 mm elbow). In both cases, ferritic steel with low chromium content was involved, with low oxygen water which favours the formation of Magnetite (Fc304), which is not very hard and more easUy attacked by the formation of soluble ferrous ions in an unfavourable water pH (<8.5 or >11). In the case of Surry 2, four casualties were caused by the explosion. 

F75 Co-Cr-Mo alloy Increased corrosion Accelerated anodic corrosion... 

Immersion Corrosion—Accelerated Tests To simulate service of components that will be immersed in fluids, ASTM G 31 should be followed. This standard includes many precautions relating to how laboratory testing environments may differ from actual service. Issues such as immersion solution composition, temperature, aeration, velocity, and volume must be addressed thoroughly before the design of the test can be considered complete. For tests involving corrosion in water, two ASTM D 2688 and ASTM D 2776 should be referenced, which determine corrosivity by weight loss and electrical methods, respectively. 

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