Contact corrosion table

The result obtained in this way are summarised in contact corrosion tables, examples of which are contained in Table 9 a)-f). The elemental current densities in particular allow for a much more useful prediction of corrosion risks in hybrid constructions compared to the practical electrochemical series. The measurements were carried out on samples with an area ratio of 1 1 and a distance of 10 mm [30, 31]. The usefulness of the values is limited for other area ratios. 

In these contact corrosion tables, the measured values are listed according to the scheme in Table 8. 

Table 9 a) Contact corrosion table in synthetic seawater... 

Table 9 f) Contact corrosion table in brackish water... 

Aufstellung von Kontaktkorrosionstabellen fur Werkstoffkombinationen in Wassern (Summary of contact corrosion tables for material combinations in waters)... 

Little comprehensive work has been carried out on contact corrosion, but some results on a range of polymers have been reported by Czech workers". In general, plastics that give rise to vapour corrosion (Table 18.18) will also cause contact corrosion. Some qualification is needed to this statement, however, as much depends on the type of contact and the other ingredients in the polymer, e.g. a paint may give good protection to the metal to which it is applied, but the vapour may cause corrosion of adjacent metal items within an enclosed space. 

The chloride ion is the most frequent cause of contact corrosion, since chlorine is present in the many chlorinated plastics, and is also frequently retained in residual amounts from reactive intermediates used in manufacture. Thus epoxides usually contain chloride derived from the epichlor-hydin used as the precursor of the epoxide. In addition to the contaminants referred to in Table 18.18, various metal and ammonium cations, inorganic anions and long-chain fatty acids (present as stabilisers, release agents or derived from plasticisers) may corrode metals on contact. 

Table 10 lists the results obtained with and without thermal stress for bolts coated with different systems [204, 239]. Because coatings on connecting elements can be damaged during assembly, which gives rise to contact corrosion, turned bolts were included in the test. 

Tables of electrochemical contact corrosion on materials used in chemical apparatus construction) (in German) Metalloberflache 26 (1972) 11, p. 413... 

The comparison of measurement results in the immersion zone of the North and Baltic Seas shows different values in the North Sea compared to the Baltic Sea. The comparison of tidal and immersion zones in the North Sea shows higher corrosion rates in the immersion zone with the exception of shipbuilding steel. In the contact corrosion tests it was seen that in the North Sea the potentials of the materials in contact with one other material are changed more than in the Baltic Sea. The mass losses per unit area of anode materials were greater in the pairings exposed in the North Sea than with the same pairings in the Baltic Sea (Table 13) [37]. Table 13 lists the factors by which the rate of corrosion is greater in contact corrosion than in free corrosion. 

Filters made of stainless steel of grade CrNil8-10 exhibited pitting corrosion due to contact corrosion with the activated carbon that is used for waste water treatment. Activated carbon is electrochemically more noble than steel. Suitable countermeasures include modification of the filter design and more frequent cleaning of the filters. The chemical composition of the steels for the outer and the itmer pipes are given in Table 19 [60]. 

Spacer would reduce the corrosion rate in the pipe. The rate of corrosion is partially determined by the difference in the standard cell potentials of the two metals in contact (see Table 9.4). The relative potential of metals in seawater is given in Table 10.1 and represents the driving force of the corrosion which includes the current, or more precisely, the current density, that is, A/cm. ... 

In general, the higher the oxidation potential the lesser the tendency to corrode. However, some metals corrode less than other metal with higher redox potential. For example, chromium (—0,74 V), zinc (—0,76 V), titanium (—0,89 V), aluminum (—1,71 V) etc. withstand corrosion much better than iron (—0,42 V). This is due to the fact that the surface of these metals coats with an insoluble very thin layer, just a veil, of hard-bitten oxide not reactive at all that, at variance with rust, passivizes the surface blocking the prosecution of corrosion. Table 13.2 provides a synoptic picture of the standard potentials, the so called electrode potential, relative to oxidation reactions of various metals. The standard electrode potential, abbreviated as , is given in volts and is the measure of the potential of any individual metal electrode which is with solute at an elfective concentration of 1 mol/dm at 1 atm of pressure. These potentials are referred to a hydrogen electrode whose reference potential is assumed equal to zero. This is because it is not possible to measure experimentally the value of the dilference of potential Ay between an electrode and its solution as, for example, in the case of zinc reaction (13.16), because any device used for making the measurement must be inserted in the circuit with two electrodes of which one is put in contact with the metal electrode of interest and the other with the solution. Now, this second electrode creates necessarily another interface metal-solution and the potential difference provided by the system is that between the two metals, without any possibility to infer the absolute value of each of them. This is why it is necessary to introduce a reference electrode, which any other potential can be referred to. To... 

Table 4 shows a galvanic series for some commercial metals and alloys. When two metals from the series are in contact in solution, the corrosion rate of the more active (anodic) metal increases and the corrosion rate of the more noble (cathodic) metal decreases. 

Dichloroethylene is usually shipped ia 208-L (55 gal) and 112-L (30 gal) steel dmms. Because of the corrosive products of decomposition, inhibitors are required for storage. The stabilized grades of the isomers can be used or stored ia contact with most common constmction materials, such as steel or black iron. Contact with copper or its alloys and with hot alkaline solutions should be avoided to preclude possible formation of explosive monochloroacetylene. The isomers do have explosive limits ia air (Table 1). However, the Hquid, even hot, bums with a very cool flame which self-extiaguishes unless the temperature is well above the flash poiat. A red label is required for shipping 1,2-dichloroethylene. 

Rhodium. Rhodium is the most commonly plated platinum-group metal. In addition to its decorative uses, rhodium has useful properties for engineering appHcations. It has good corrosion resistance, stable electtical contact resistance, wear resistance, heat resistance, and good reflectivity. The use of rhodium for engineering purposes is covered by an ASTM specification (128). Typical formulas are shown in Table 15. The metal content is obtained from prepared solutions available from proptietary plating supply companies. Replenishment is requited because anodes are not soluble. Rhodium for decorative use may be 0.05—0.13 p.m thick for industtial use, it maybe 0.50—5.0 p.m thick. 

Galvanic Corrosion Galvanic corrosion is the corrosion rate above normal that is associated with the flow of current to a less active metal (cathode) in contact with a more active metal (anode) in the same environment. Tables 28-1 7 and 28-li show the galvanic series of various metals. It should be used with caution, since exceptions to... 

Generai description. Galvanic corrosion refers to the preferential corrosion of the more reactive member of a two-metal pair when the metals are in electrical contact in the presence of a conductive fluid (see Chap. 16, Galvanic Corrosion ). The corrosion potential difference, the magnitude of which depends on the metal-pair combination and the nature of the fluid, drives a corrosion reaction that simultaneously causes the less-noble pair member to corrode and the more-noble pair member to become even more noble. The galvanic series for various metals in sea water is shown in Chap. 16, Table 16.1. Galvanic potentials may vary with temperature, time, flow velocity, and composition of the fluid. 

The switching-off method for 7/ -free potential measurement is, according to the data in Fig. 3-5, subject to error with lead-sheathed cables. For a rough survey, measurements of potential can be used to set up and control the cathodic protection. This means that no information can be gathered on the complete corrosion protection, but only on the protection current entry and the elimination of cell activity from contacts with foreign cathodic structures. The reverse switching method in Section 3.3.1 can be used to obtain an accurate potential measurement. Rest and protection potentials for buried cables are listed in Table 13-1 as an appendix to Section 2.4. The protection potential region lies within U[[Pg.326]

It can be seen from Table 1.16 that differences in composition (nature or concentration) of the environment can lead to localised attack, and in Section 1.4 it was shown how differences in the activity of silver ions can give rise to a reversible concentration cell in which the silver electrodes in contact with the solution containing the lower and higher concentration of Ag ions are the anode and cathode, respectively. Concentration cells of this type are rare in practice, but can occur during the corrosion of copper and copper alloys. 

Table 1.25 is reproduced from Corrosion and its Prevention at Bimetallic Contacts (H.M.S.O., London, 1958) by permission of the Director of Publications. The reader should note that it is recommended that the Table be used only in conjunction with the Introduction to the original publication. 

Protective measures against bimetallic corrosion should ideally start before the particular installation or equipment is built . Reference should be made to tables showing compatibility of metals, alloys and non-metallic materials (5 Table 1.25) and to the literature. However, it must be emphasised that the environment obviously plays a most important role in bimetallic corrosion, and that there are a number of situations in which apparently incompatible materials in contact can be used without adverse effects. 

Table I shows the chemical composition limits of various aluminum alloys presently used for packaging applications (3). In general, these alloys have good corrosion resistance with most foods. However, almost without exception, processed foods require inside enameled containers to maintain an acceptable shelf life (4, 5). Moreover, when flexible foil packages are used for thermally processed foods, the foil is laminated to plastic materials that protect it from direct contact with the food and also provide heat sealability as well as other physical characteristics (6,7).

If dissimilar metals are placed in contact, in an electrolyte, the corrosion rate of the anodic metal will be increased, as the metal lower in the electrochemical series will readily act as a cathode. The galvanic series in sea water for some of the more commonly used metals is shown in Table 7.4. Some metals under certain conditions form a natural protective film for example, stainless steel in oxidising environments. This state is denoted by passive in the series shown in Table 7.4 active indicates the absence of the protective film. Minor... 

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