Corrosion currents, causes

The anodes can be made of graphite which tolerates high current densities without passivation, but are subject to gradual corrosive attack causing a... 

The Journal fur Gasbeleuchtung mentions electrolytic corrosion damage caused by direct current cables in Berlin in 1892, and a few years later damage by tramway currents was reported in 14 German towns. As early as 1894 the electrolytic processes of stray current corrosion were explained in detail in this Journal by G.Rasch [65]. 

In Sec. 4.3.3 it has been shown that corrosion is one or the reactions that cause selfdischarge of the positive electrode. In connection with Fig. 8 it has been mentioned that an anodic current, the corrosion current, must flow continuously to stabilize the lead dioxide layer at the grid surface. Then the PbOA layer remains thin because PbOr is always converted into Pb02 by... 

Fig. 5.54 Mixed potential. (A) Zinc dissolution in acid medium. The partial processes are indicated at the corresponding voltammograms. (B) Dissolution of mercury in nitric acid solution. The original dissolution rate characterized by (1) the corrosion current y a is enhanced by (2) stirring which causes an...

Figure 1. Schematic representation of potential profile and charge distribution across an anodic oxide film of thickness S on aluminum (a) hypothetical situation in the absence of any current (b) in the presence of an anodic current caused by corrosion or by an external source. RE, reference electrode to which the potential of aluminum is referred.

All metals will corrode under certain conditions. Internal corrosion is caused by galvanic corrosion, pitting, corrosion fatigue, stress corrosion cracking, stray currents, etc. 

The problem with relying solely on anodic area corrosion inhibition is the risk of local film damage, which concentrates the corrosion current flow and permits a highly active anodic cell to be developed and causing accelerated corrosion to take place. This in turn leads to severe metal wastage, often in the form of deep pitting. 

The two-dimensional, coupled kinetic and transport model can also be used to simulate start-stop processes. Figure 14 plots cathode potential and carbon corrosion current distribution at three instants when the H2/02 front passes through 10, 50, and 90% of anode flow path during the start process. As H2 displaces air in the anode flow-field, the size of the power source increases and the load size decreases accordingly. The balanced current density becomes larger, causing higher carbon corrosion current density. 

A ballast tank filled with seawater is easily corroded. Corrosion protection by the paint on the metal surface inside the tank, which improves the insulation for the corrosion current, is conducted. The paint has problems with age-related degradation and incipient failure. To protect from the corrosion caused by these problems, plural sacrificial anodes are usually installed in the tank. When seawater is loaded in the tank, the surface of the inside tank becomes cathode and the protective potential works, because of the anode effects. The worse the coating condition becomes, the worse the insulation of the paint becomes and the lower the surface resistance becomes. Therefore, there is the possibility that the coating condition can be evaluated with the monitoring of the surface resistance. 

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

Most corrosionists agree on the fact that corrosion current density is a very important parameter for the evaluation of the kinetics of a corrosion process and the proper choice of a metal to be used in a given environment with no prejudice to its integrity and performance. Hence it is very interesting to examine analytically the influence of the ohmic drop on the determination of the corrosion rate. In fact, this analysis makes it possible to detect a priori situations that may cause the behaviour of an electrochemical system to diverge from its ideal trend and render the use of equation (10) mandatory for a more reliable evaluation of the kinetics of the corrosion process. 

All specimens display heavy corrosion at the fault site in the lower region of the cruciform injury site, they show formation of rust smears which tend to spread in the direction of the current caused by the stirrer. 

Materials problems in newer coal gasification processes accrue generally from operating temperatures of 1500 -2800°F and pressures of 150-1200 psi. Added to these conditions are low oxygen activity and high sulfur activity in the product gas atmosphere. Finally, coal ash and sulfur sorbents present in the system can cause materials failure by corrosion and/or erosion-corrosion. Current metallic alloys that were developed... 

Fig. 10 Stray current effects in underground pipelines (a) stray currents cause corrosion in neighboring pipelines (b) redesign minimize stray current effects [39].

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