Contact corrosion, 6.37

The metal is extensively used in jewelry, wire, and vessels for laboratory use, and in many valuable instruments including therocouple elements. It is also used for electrical contacts, corrosion-resistant apparatus, and in dentistry. 

The attraction of rubbed amber and some other effects of electricity were known in ancient times. We know from finding nails in an old wreck that the Romans knew about contact corrosion combined with electric current flow. A skin of lead as a protection against boring worms covered the wooden planks of the ship and was nailed down with copper nails. Galvanic coupIe.s formed between the lead and the copper nails and the less noble lead sheets around the nails corroded in the seawater and fell off. The shipbuilders discovered a simple solution and covered the heads of the copper nails with lead as well. Galvanic current flow between the two metals was eliminated and corrosion was prevented (26). 

The aluminium coatings are highly corrosion resistant, and are less liable to contact corrosion than cadmium when in contact with light alloys. They can also be used at temperatures of up to 496°C as against 232°C for cadmium. The aluminium coating is also unaffected by aviation fuels, unlike cadmium. 

Packaging materials Materials to be used in contact with metals should be as free as possible from corrosive salts or acid. BS 1133, Section 7 1967 gives limits for non-corrosive papers as follows chloride, 0-05% (as sodium chloride) sulphate, 0-25% (as sodium sulphate) and pH of water extract 5 -5-8 0. Where there is doubt, contact corrosion tests may be necessary in conditions simulating those in the package. 

Although less widely reported than the effects of vapours, contact corrosion has been a serious problem in packaging and in electronicsAs miniaturisation and sophistication of electronic devices has increased, the hazard presented by corrosion is often the limiting factor inhibiting the attainment of expected levels of reliability. 

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. 

Contact corrosion may be reduced by the presence of natural inhibitors, such as tannins, in the wood, and will be promoted by sulphates and chlorides in it, especially if mineral preserving processes involving these ions have been applied. 

Contact corrosion Nails and fastenings in many non-durable wooden articles exposed to damp will outlive the useful lives of the articles, and their corrosion is of no great importance. Corrosion is, however, important in tile... 

Poor choice of FR additives can lead to excessive loss of PBT molecular weight upon processing, hence leading to impaired mechanical properties (usually seen as part melt-viscosity (MV) drop and, if severe, part breakage). In some cases, generation of acidic halide species can cause mold or electrical contact corrosion. 

Dissimilar-metal corrosion can occur even if the two metals are not initially in direct contact. For example, in homes where copper tubing is used for plumbing, there is always a small amount of dissolved Cu2+ in the water. When this water encounters steel piping or a chrome-plated bathroom sink drain, the more-noble copper will plate out on the other metal, producing a new metals-in-contact corrosion cell. In the case of chrome bathroom sink fittings, this leads to the formation of Cr3+ salts which precipitate as greenish stains. 

Procedures for corrosive materials minimize risk of contact Corrosive materials destroy or permanently change living tissue through chemical action. Irritants cause inflammation due to an immune response but not through chemical action. The effect is usually reversible but can be severe and long lasting. Sensitizers are irritants that cause no symptoms after the first exposure but may cause irritation during a later exposure. 

SAFETY PROFILE Poison by ingestion. Moderately toxic by inhalation and skin contact. Corrosive to skin. A mild allergen. Symptoms of exposure are sweating, flushing, shormess of breath, and slow pulse. Combustible when exposed to heat or flame can react with oxidizing materials. To fight fire, use foam, CO2, dry chemical. When heated to decomposition it emits toxic fumes of NOj. See also AMINES. 

SAFETY PROFILE Poison by inhalation. Moderately toxic by ingestion and skin contact. Corrosive. When heated to decomposition it emits very toxic fumes of Cr, POx, and SOx. 

Electrochemical methods, such as potentiodynamic techniques and corrosion current measurements, can be applied [99] to simulate atmospheric contact corrosion... 

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. 

Zirconium-silicate refractories have found successful application in certain parts of glass furnaces, in particular those for borosilicate and fluoride glasses (tank, feeder, throat). Refractories based on Cr203 have recently been employed at the critical points of glass tanks. This oxide show s the best resistance to glass-contact corrosion but its application is limited by its colouring effect. 

Risks Harmful by ingestion, inhalation and skin contact. Corrosive to skin and eyes. Frequent exposure can cause ulceration, liver and kidney disease and cancer. Do not eat drink or smoke while handling this material - wash hands and face after handling. 

HEALTH SYMPTOMS inhalation (eye and skin irritation) contact (corrosive on skin, dermatitis, burning). 

HEALTH SYMPTOMS inhalation (pulmonary edema, irritates eyes, skin and respiratory system) contact (corrosive to body tissues, causes severe eye and skin bums). 

Specimen 7 displays irregular corrosion spread over the entire surface of the specimen. It takes the form of wide depressions with pitting corrosion. The test solution is rust-brown and cloudy with a rust-coloured sediment, the main component of which is ferric hydroxide. This picture is typical of contact corrosion between unalloyed steel and copper in sodium chloride solutions (Figure 11.85(g)). 

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