Protection against corrosion

Corrosion experts proclaim, and they are right in doing so, that aluminium resists corrosion well. Why is it nevertheless necessary to protect it against corrosion The answer to this question depends on the alloy, applications and use. 

It is well known that copper-containing alloys of the 2000 and 7000 series are not suflhciently corrosion-resistant to be used without protection in humid media such as ambient atmosphere, and all the more in aggressive media such as marine environments. 

Incidentally, it was for the protection of aircraft in Duralumin that chemists looked for surface treatments in the 1920s. Anodising, invented by Bengough and Stuart in 1923, was a response to this need. 

On the other hand, strain-hardenable alloys of the 1000, 3000 and especially 5000 series, as well as age-hardenable alloys of the 6000 series have good corrosion resistance. They can, therefore, be used without protection in many environments such as maritime environments. Many vessels in aluminium are painted on the visible parts and for decorative purposes only. Many small crafts in aluminium are not painted at all. The same is true for landing stages of marinas, road sign supports, etc. Water staining and superficial pitting corrosion do not affect the solidity of the component. 

There are several ways to prevent corrosion of a metal  

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Intermetallic compounds with gallium are used as semiconductors. Indium is used to coat other metals to protect against corrosion, especially in engine bearings it is also a constituent of low-metal alloys used in safety sprinklers. The toxicity of thallium compounds has limited the use of the metal, but it does find use as a constituent of high-endurance alloys for bearings. 

Although methylene chloride is considered a very stable compound, small amounts of stabilizets ate usually added at the time of manufacture. Additional stabdizets may be used to provide adequate protection against corrosion or solvent breakdown in specific appHcations. A representative commercial grade of methylene chloride has the following specifications ... 

An important function of many coatings is to protect metals, especially steel, against corrosion. Corrosion protection is required ia two different situations ia one case, the steel is protected against corrosion with iatact coating films ia the other case, the objective is to protect the steel against corrosion even when the film has been mptured. 

A frequently cited example of protection from atmospheric corrosion is the Eiffel Tower. The narrow and, for that age, thin sections required a good priming of red lead for protection against corrosion. The top coat was linseed oil with white lead, and later coatings of ochre, iron oxide, and micaceous iron oxide were added. Since its constmction the coating has been renewed several times [29]. Modern atmospheric corrosion protection uses quick-drying nitrocellulose, synthetic resins, and reaction resins (two-component mixes). The chemist Leo Baekeland discovered the synthetic material named after him, Bakelite, in 1907. Three years later the first synthetic resin (phenol formaldehyde) proved itself in a protective paint. A new materials era had dawned. 

Due to both carbonization and penetration of chloride ions, steel will pass from a passive to an active condition and (consequently) may corrode. If the mortar is completely surrounded by water, oxygen diffusion in wet mortar is extremely low so that the situation is corrosion resistant because the cathodic partial reaction according to Eq. (2-17) scarcely occurs. For this reason the mortar lining of waste pipes remains protective against corrosion even if it is completely carbonated or if it is penetrated by chloride ions. 

Buried steel pipelines for the transport of gases (at pressures >4 bars) and of crude oil, brine and chemical products must be cathodically protected against corrosion according to technical regulations [1-4], The cathodic protection process is also used to improve the operational safety and economics of gas distribution networks and in long-distance steel pipelines for water and heat distribution. Special measures are necessary in the region of insulated connections in pipelines that transport electrolytically conducting media. 

The surfaces to be protected should be the total surface, including inserts, spars and pipes. The upper 1.5 m of the side walls and the covers should be provided with a coating of recognized quality [10] to protect against corrosion. 

No one, it seems, realized the importance of the supports. Unlike the pipework, they were not protected against corrosion and were removed with little or no thought about the consequences [23]. 

Korrosions-mittel, n. corrosive, -schutz, m. protection against corrosion, -wirkung, /. corrosive effect. 

To have the fan represent the best possible selection considering the particular circumstances and requirements, it is important to study the fan type curves and to recognize whether a small change in system resistance would be easily handled by a particular fan, whether speed variations and the resulting volume and pressure changes are acceptable, and whether the fan can be protected against corrosion, etc. References 19, 31, and 38 will be helpful. Specifications should be submitted to several manufecturers for their recommendations. In this way full advantage is received from... 

Metal components of anchors should be galvanized or otherwise protected against corrosion. Sucker rods should not be used in anchor construction. Anchor location should be marked with a stake if projections aboveground are subject to bending or other abuse. 

Buried pipework must be protected against accidental and physical damage from sharp material, etc. and chemical action from corrosive soils, etc. It must be protected against corrosion by means of wrapping, catholic protection, etc. for metal pipes. Above-ground pipework should be protected with suitable paint after preparation. 

Even today, msting of industrial plant and material is accepted by some as an inevitable operating expense. There is no necessity for this attitude, however, as the petroleum industry has evolved effective, easily applied temporary protectives against corrosion, which are well suited to the conditions met in practice. 

Nickel is also widely used as an electrodeposited underlay to chromium on chromium-plated articles, reinforcing the protection against corrosion provided by the thin chromium surface layer. Additionally the production of articles of complex shape to close dimensional tolerances in nickel by electroforming —a high-speed electrodeposition process —has attracted considerable interest. Electrodeposition of nickel and the properties of electro-deposited coatings containing nickel are dealt with in greater detail in Section 14.7. 

While plasma deposits are widely used, especially in the American aero industry to provide wear resistance, there is not at the moment any great demand for the exotic materials deposited to be used as a protection against corrosion. However, M. A. Levinstein of General Electric (USA) reports the successful use of sprayed chromium carbide as a protection for ventilator blades operating in corrosive conditions. 

Methods of Protection Against Corrosion for Light Gauge Steel Used in Building, PD 420 (1953)... 

The anodic oxidation of magnesium does not normally produce a film that has sufficient corrosion resistance to withstand exposure without further protection by painting, and the solutions used are complex mixtures containing phosphates, fluorides and chromates. In the case of aluminium, a relatively simple treatment produces a hard, compact, strongly adherent film of oxide, which affords considerably increased protection against corrosive attack . 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

Temporary protectives against corrosion should be used only where removal is subsequently necessary for the fitting or the working of surfaces to which they are applied. 

The toxicity of lead-containing greases has led to alternative products being used for the protection of components where the product is likely to come in contact with rubber. Of those products considered silicone-based greases have been found to be particularly suitable and their application to hydraulic equipment components such as brake cylinders, where they can provide internal protection against corrosion both during transit and use, has been found particularly beneficial. 

Filming amines In such situations, one approach is to use the filming type of amine, of which octadecylamine is the most common example. This, and related substances, forms a barrier on the internal surfaces of the feed system offering some protection against corrosive attack. 

Pipes buried in the structural slab. These are connected to delivery and return headers, and glycol circulated. This is heated by waste heat from the refrigeration plant. Steel pipe should not be used under the floor unless protected against corrosion. Air vent pipes to allow a current of ambient air through the ground under the base slab. This is not very suitable in cold climates. 

Distribution Ratio (DR) The DR relates the concentration of an amine present in the steam phase to that concentration in the condensate phase (vapor-liquid distribution ratio). Consequently, it identifies in which condensate production region of a steam-condensate system any particular amine will concentrate and thus provide protection against corrosion. It also helps to indicates the portion of amine loss due to vaporization in a condenser or venting of a deaerator. The expression for DR is shown here ... 

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