Corrosion-resistance

The corrosion resistance of reinforced plastics is very dependent upon the type of resin used. As with fire resistance, only indicative information is described herein and more detailed information must be sought from databases and suppliers. 

(1986) Design Data Fibreglass Composites Owens Corning Fiberglas, Battice. 

The chemical resistance of a polyester resin depends upon the constituents from which the resin is manufactured. A general classification is given in Table 9.6. 

Not resistant with the exception of iso-NPG, bisphenol and vinyl ester. Acids 

Resistant except at high concentrations in some instances or where the acid is a strong oxidant like H2SO4 and HNO3 above certain concentrations and at elevated temperatures. 

The conditions that cause corrosion can arise in a variety of ways. For this brief discussion on the selection of materials it is convenient to classify corrosion into the following categories  

Metallic corrosion is essentially an electrochemical process. Four components are necessary to set up an electrochemical cell  

Refractories are considered to be corrosion-resistant to melts and gases in furnaces and high-temperature devices. As mentiOTied in Sect. 1.6, approximately one third of refractories are damaged because of poor thermal shock resistance at temperatures sufficiently lower than refractoriness. Most likely, this ratio for the refractories in the A1 industry and the decay of the refractories due to corrosiiMi are even higher. 

The corrosion includes the process of chemical corrosion itself, but it is also necessary to take into accoimt the penetration of the aggressive liquid in open permeable pores, the diffusion of the aggressive melt (or constituents of the aggressive melts) into the structure of refractory materials, and others. 

The reactions of gases and liquids with refractories may cause stresses, resulting in cracking and spalling, and open the door to the penetration of aggressive liquids. 

In ferrous metalltugy, the corrosion resistance of refractories has been investigated for more than 100 years. The general principles of refractory corrosion approaches have been described by Brosnan, Rigaud, and several other authors [116-118]. 

Yet the Al industry has several peculiarities. Relatively low temperatures in reduction cells and in the cast house are compensated by the high corrosion ability of molten aluminium and the bath. The main constituent of the bath for the electrolysis process, N3AIF6, has a unique ability to dissolve alumina - the constituent of the major part of the refractories. However, it also dissolves other oxides very quickly. Refractories for Al came from refractories for the ferrous industry. 

Austenitic cast irons (either flake graphite irons or nodular graphite irons) are produced by mixing in nickel from 13-30%, chromium from 1-5% and copper from 0.5-7.5 (to lower nickel-containing grades to augment the corrosion resistance at lower cost). 

The main advantages of austenitic cast irons are corrosion and heat resistance. For corrosion resistance, the flake and nodular are similar, but the mechanical properties of nodular cast irons are superior. Some of the commercially available austenitic cast irons are given in the Tables 3.4 and 3.5. 

The white cast irons and their low alloys have good abrasion resistance properties [2,3]. White cast irons are used for grinding balls, segments for mill liners and slurry pumps. In the ceramic industry they are used for muller tyres and augers in the pulp and paper industry for attrition mill plates and chip feeders and in the paint industry for balls for grinding pigments. 

The corrosion resistance of unalloyed and low-alloy flake, nodular, malleable and white cast iron is comparable to mild- and low-alloy steel. However, these cast irons have a major advantage over steel namely, greater cross section or wall thickness than steel. Consequently, they have a 

BS3468 Designation astm A439 Designation Trade Names C (% max.) Si (%) Mn (%) 

In carbonated concrete, or in the case where the concrete is extensively cracked, the critical chloride contents are remarkably lower. Situations where carbonated concrete and high chloride levels are simultaneously present are rare, but can be found, for instance, inside road tunnels [29]. The more highly alloyed stainless steels should be preferred in these more aggressive conditions. It is well known that for austenitic and duplex stainless steel, an increase in the content of chro- 

PRE is also a vahd parameter when stainless-steel parts are not completely embedded in concrete and are partially in direct contact with the aggressive environment. In these cases it is more appropriate to specify austenitic stainless steels with molybdenum, since they provide additional corrosion resistance. A minimum molybdenum content of 2.5% is preferable to 2%, because of the resulting increase in corrosion resistance with only marginal increase in cost. 

The critical chloride content decreases as temperature increases for instance Eigure 15.3 shows the expected variations between 20 °C and 40 °C. Thus bars made of steels containing molybdenum should be preferred in hot cHmates. 

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FEP plastics Copolymers of tetrafluoro-ethene and hexafluoropropene. Inert and corrosion resistant as Teflon but can be processed by melt techniques. 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

For example,copper has relatively good corrosion resistance under non-oxidizing conditions. It can be alloyed with zinc to yield a stronger material (brass), but with lowered corrosion resistance. Flowever, by alloying copper with a passivating metal such as nickel, both mechanical and corrosion properties are improved. Another important alloy is steel, which is an alloy between iron (>50%) and other alloying elements such as carbon. 

Other authors have attributed the improved corrosion resistance with increasing Cr content with the increasing tendency of the oxide to become more disordered [69]. This would then suggest that an amoriDhous oxide film is more protective than a crystalline one, due to a bond and stmctural flexibility in amoriDhous films. 

It is extensively used for making stainless steel and other corrosion-resistant alloys such as Invar(R), Monel(R), Inconel(R), and the Hastelloys(R). Tubing made of copper-nickel alloy is extensively used in making desalination plants for converting sea water into fresh water. 

Pure vanadium is a bright white metal, and is soft and duchle. It has good corrosion resistance to alkalis, sulfuric and hydrochloric acid, and salt water, but the metal oxidizes readily above 660oC. 

Titanium, when pure, is a lustrous, white metal. It has a low density, good strength, is easily fabricated, and has excellent corrosion resistance. It is ductile only when it is free of oxygen. The metal, which burns in air, is the only element that burns in nitrogen. 

It is used in certain nickel-based alloys, such as the "Hastelloys(R)" which are heat-resistant and corrosion-resistant to chemical solutions. Molybdenum oxidizes at elevated temperatures. The metal has found recent application as electrodes for electrically heated glass furnaces and foreheaths. The metal is also used in nuclear energy applications and for missile and aircraft parts. Molybdenum is valuable as a catalyst in the refining of petroleum. It has found applications as a filament material in electronic and electrical applications. Molybdenum is an... 

Because the element not only has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), but also excellent mechanical properties and is extremely corrosion-resistant, hafnium is used for reactor control rods. Such rods are used in nuclear submarines. 

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. 

It is the most corrosion-resistant metal known, and was used in making the standard meter bar of Paris, which is a 90 percent platinum and 10 percent iridium alloy. This meter bar was replaced in 1960 as a fundamental unit of length (see under Krypton). 

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