Journal of the Royal Naval Scientific Service. Volume 27, Number 2 ...

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II MATERIALS FOR EXTREME ENVIRONMENTS J. F. G. Conde, B.Sc, F.I.M., R.N.S.S. and D. J. Godfrey, B.Sc, Ph.D., R.N.S.S. Admiralty Materials Laboratory The development of modern Introduction technology in the past 30 years has placed increasing demands upon the technologist and materials scientist for materials which can withstand the extreme environmental conditions in which they will operate. Economic factors have placed additional demands on the ingenuity of both the materials technologist and the designer and fabricator. The major areas where materials evolution has been most dramatic are resistance to corrosion and to high and cryogenic temperatures and in nuclear reactor materials. The economic implications of corrosion have been highlighted by a recent Government sponsored survey which has put the cost of corrosion in the U.K. at greater than £1600 million per annum. In this connection it should be noted that the natural environment is one of the most severe in relation to corrosion. The technology of high and low temperatures and nuclear reactors has developed mainly within the past 30 years and it seems 'likely that in the next 30 years this trend will continue with perhaps growing emphasis on cryogenics. There are very few engineer- Corrosion in ing metals which are immune Saline to corrosion in sea water. Steel Environments has a corrosion rate of 150,11m/ yr, and copper 50^m/yr, the latter being acceptable for many applications. Zinc coating of steeel is an effective means of protection, since the corrosion of zinc is about 30/xm/yr in sea water, and also allows steel to be used with aluminium without causing serious galvanic corrosion. More corrosionresistant copper alloys such as Cu, 10 Ni, 1 Fe and Cu, 30 Ni, are used where water velocities are sufficient to cause impingement attack, and aluminium brass where good heat transfer is required. These materials are widely used in heat exchangers, although very occasionally trouble is encountered in polluted or anaerobic H 2 S. bearing waters. Aluminium bronzes formulated to resist de-aluminification selective attack with nickel or silicon additions are used when high strengths are required. Gun-metals, Cu/Sn alloys with zinc and sometimes lead additions are used where good castability and corrosion-resistance are both necessary. It might be thought that stainless steels would
112 J.R.N.S.S., Vol. 27, No. 2 be the answer to corrosion problems in sea water, in view of their excellent behaviour in many other environments, but differential aeration local attack or " crevice corrosion " can be a very serious problem. Although worst with martensitic/ferritic 13% Cr steels, none of the austenitic materials, 18 Cr, 8 Ni, etc is immune to crevice corrosion, and these materials are not suited for general service immersed in sea water, although there are a few well-established uses where crevice corrosion has not proved to be a difficult problem. A few highly alloyed and rather expensive materials, such as Inco 625 (60 Ni, 21 Cr, 9 Mo, 5 Fe, Nb + Ta 3 5) combine high strengths and immunity to corrosion, including crevice corrosion, and are used when any corrosion is unacceptable. Titanium and its alloys are practically immune 'to attack in sea water, but their relatively high cost has limited their use. Aluminium and some Al/Mg alloys (up to 5% Mg) are quite resistant to sea water. Stronger alloys (Al/Si/Mg) have slightly greater corrosion rates in saline environments and are significantly stronger. In general, very high strength Al alloys have unacceptable corrosion behaviour, and sometimes suffer from stress corrosion. Used in combination with other metals, serious galvanic corrosion of aluminium can occur, and even deposition of dissolved copper from solution can cause this. Another environment resembling sea water and the severity of its corrosion problems is the tissue fluid of animals. The use of certain stainless steels here is accepted practice, although crevice corrosion and galvanic interaction between different steels have occasionally been encountered. Vitallium, a Co/Cr alloy, has been used successfully for many years and titanium has shown promise. One problem is that polymeric materials, which in many cases are satisfactory in saline environments, tend to undergo enzymatic degradation, and it has been claimed that there is some effect on every material and that such attack, e.g. on certain polyurethanes, can be severe. Corrosion in Atmospheric and Fresh Water Environments Corrosion of materials in inland atmospheric exposure and fresh water is less severe than in marine conditions. The action of SC>. and acidic dust particles has been shown to be responsible for the corrosivity of industrial atmospheres for ferrous materials, but corrosion can be controlled successfully with effective paint or polymer coatings or sacrificial metal coatings, such as zinc, aluminium and cadmium. The use of stainless steel and aluminium is well-established. The choice of materials with fresh water depends upon the corrosivity of the water, which is controlled in a complex manner by the total dissolved salts, its magnesium and calcium content and C0 2 and organic material content. Cast iron, copper and gallvanised coatings may be attacked in various local waters and dezincification of brasses, a severe problem in sea water, can be encountered in certain fresh waters. Although complex, these problems are now well understood, but informed advice is necessary when materials are selected. Materials for High Temperature Environments Owing to oxidation, creep and loss of strength, the use of steel becomes difficult at high temperatures. Special alloys and coatings. e.g. sprayed Al, diffused Cr, allow use to higher temperatures but ultimately alloys of high non-ferrous content have to be used. Chromium additions confer the best oxidation resistance, but nickel lowers the thermal expansion coefficient of the alloy, thus improving the behaviour in thermal cycling. The best oxidation resistance at temperatures above 5-600° is given by materials with higher alloy contents than the 18 Cr, 8 Ni, steels, e.g. 310 (25 Cr, 20 Ni) or 330 (15 Cr, 35 Ni, 1 Si) types. Optimum performance at the highest temperature at which use of metal* is practicable (900 - 1,150°C) is given by nickel chromium alloys (80 Ni, 20 Cr - the original " Nimonic " ) or iron containing alloys such as Incoloy 800 (32 Ni, 21 Cr) and Hastefloy X (50 Ni, 21 Cr, 18 Fe, 9 Mo, 1 Co). Other nickel and cobalt alloys, developed to yield optimum mechanical and creep resistance properties at temperatures up to 900°C, also have good oxidation resistance. The presence of SO.,, C0 2 , FLO and chloride in general tend to increase attack, sometimes up to ten times. Sulphur can cause serious penetration of the metal beneath the oxide scale, especially if ch'loride is present, and increasing Cr content is a promising remedial measure. At temperatures above 1,000°C the use of ceramics may be considered. Alumina is useable, depending on purity, up to 1,800°C, although care has to be taken to avoid thermal stress failure and
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