Galvanic corrosion conditions

In several eases, non-metallie materials such as polymers, rubbers, ceramics, wood or concrete must also be taken into consideration (see Section 10.1.11). With respect to materials selection for screws, bolts, nuts, rivets or oflier small parts for use under possible galvanic corrosion conditions, it should be checked that they are a little more noble (have a little more positive corrosion potential in the actual enviroiunent) than the components they are binding together, or are in metallic contact with in any way. In addition, one should be particularly careful to avoid catastrophic deterioration forms such as hydrogen embrittlement, stress corrosion cracking and corrosion fatigue in such parts. 

The fact that some materials are more easily oxidized than iron provides a way to construct galvanic corrosion conditions intentionally to protect the iron. If we choose a metal, such as magnesium, whose reduction potential is more negative than that of iron, the magnesium is oxidized, and iron is reduced ... 

Corrosion tests of metals under static conditions reveal nothing relating to erosion-corrosion susceptibilities. It is entirely possible that a metal tested under static conditions will fail in service when sufficient fluid velocity produces erosion-corrosion. Similarly, it has been observed that galvanic corrosion between coupled, dissimilar metals may be accelerated or even initiated under flow conditions when little or no galvanic corrosion is observed under static conditions (see Chap. 16, Galvanic Corrosion ). 

The necessary conditions for galvanic corrosion are (1) a corrosive interaction of electrochemically dissimilar materials that are (2) exposed to a common conductive fluid and are (3) physically linked so... 

The galvanic series in Table 16.1 is generally useful for predicting the tendency toward galvanic corrosion between coupled metals. The arrangement of this series is, however, based on data generated under controlled laboratory conditions on clean, bare metals. 

The possible effects of fluid velocity on galvanic corrosion are sometimes overlooked. Fluid velocity can affect the apparent potential of metals in a given environment. Depending on the environment, a metal under the influence of relatively rapid flow may assume either a more noble or a more active character than that indicated by the galvanic series. Occasionally, this shift in potential may result in galvanic corrosion that would not occur under stagnant or low-flow conditions. 

Avoid the common tendency to attribute to galvanic corrosion the deterioration of one metal simply because another metal is nearby. The conditions necessary for galvanic corrosion are specific, and all must be operating simultaneously for it to occur. These conditions are outlined in the General Description section of this chapter. 

If conditions are such as to require a duplex tube, it is quite likely that a plain end detail for the tube will not be satisfactory. Grooved or serrated joints are recommended for this type of tube, and the ends should be flared or beaded. Table 10-8 gives recommended flare or bell radii for copper-based alloys. Also see Table 10-8A. In service where galvanic corrosion or other corrosive action may take place on the outside material used in the tube, a ferrule of inside tube... 

Except for fluid conditions of possible galvanic corrosion, the tins can be any selected material, not necessarily the same as the tube. Some usable tin and/or tube materials are... 

An obvious method for studying galvanic corrosion either with or without supplementary electrical measurements is to compare the extent of corrosion of coupled and uncoupled specimens exposed under identical conditions. Such measurements may use the same techniques for estimating corrosion damage, such as mass-loss determinations, as have been described in connection with ordinary corrosion tests. 

The real electrical potential of various metals and their alloys may, under practical boiler operating conditions, be considerably different from their standard potential under ideal conditions. Thus, a reversal of potential may take place in the boiler plant system, with unexpected forms of galvanic corrosion occurring. 

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

If dissimilar materials with significantly different eiectrochemical potentials are placed in contact in the presence of an electrolytic solution, galvanic couples can be created that can result in serious corrosion of the less noble material. The vendor shall select materials to avoid conditions that may result in galvanic corrosion. Where such conditions cannot be avoided, the purchaser and the vendor shall agree on the material selection and any other precautions necessary. The NACE Corrosion Engineer s Reference Book is one source for selection of suitable materials in these situations. 

Since two dissimilar metals are in contact galvanic corrosion may be suspected. When two dissimilar conducting materials in electrical contact with each other are exposed to an electrolyte, a galvanic current flows from one to the other. Galvanic corrosion is that part of the corrosion that occurs at the anodic member of the couple and is directly related to the galvanic current by Faraday s Law. Under a coupling condition, the simultaneous additional corrosion taking place on the anode of the couple is known as the local corrosion. 

Coupling of dissimilar metals in the atmosphere may also result in galvanic corrosion. Figure 1.22 can be used to determine the compatibility of metals when exposed to atmospheric conditions that cause corrosion (i.e., when the relative humidity exceeds -50%). 

In this paper, an inverse problem for galvanic corrosion in two-dimensional Laplace s equation was studied. The considered problem deals with experimental measurements on electric potential, where due to lack of data, numerical integration is impossible. The problem is reduced to the determination of unknown complex coefficients of approximating functions, which are related to the known potential and unknown current density. By employing continuity of those functions along subdomain interfaces and using condition equations for known data leads to over-determined system of linear algebraic equations which are subjected to experimental errors. Reconstruction of current density is unique. The reconstruction contains one free additive parameter which does not affect current density. The method is useful in situations where limited data on electric potential are provided. 

Situations may arise, where the boundary conditions are unknown and only some experimental data in certain locations are known. In this case, the problem is defined as an inverse one. This situation often occurs in many branches of science and mathematics where only the values of some model parameters can be obtained from observed data or measured data. Data on electric potential can be obtained in galvanic corrosion as a set of discrete data with one free parameter due to measuring potential differences. This situation, where measurements on electric potential can be provided as a set of discrete data within simply connected domain Q imposes the problem to be inverse. 

Galvanic corrosion is of particular concern in design and material selection. Material selection is important because different metals come into contact with each other and may form galvanic cells. Design is important to minimize differing flow conditions and resultant areas of corrosion buildup. Loose corrosion products are important because they can be transported to the reactor core and irradiated. 

All of the correlations seen above refer to situations of steel reinforcement in the free corrosion condition, that is, in the absence of factors that modify the potential of the system. They are in particular not appHcable to structures in concrete containing corrosion inhibitors galvanized reinforcement (on stainless steel it is possible in the same way) structures subjected to electrical fields produced by stray current that induce current exchange between reinforcement and concrete (this case is dealt with in Section 9.4). 

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