A MAJOR FACTOR in determining the specific applications of tantalum for corrosion-resistant equipment is its price. The cost of tantalum metal is about 15 times that of nickel-copper, nickel, or titanium and about 50 to 100 times that of the stainless steels. Accordingly, the service requirements for tantalum components are to have practically no corrosion degradation over time periods of years-virtually indefinite life. This means that the use of tantalum is limited to a relatively small number of the most severe media and conditions in relatively thin cross sections or as lining compared to other materials of construction. Selection of tantalum for use is based on large, not moderate, improvements of performance over other materials. Therefore, testing data do not need to be highly quantified. If corrosion rates greater than 0.051 mm/yr. (2 mpy) are indicated the application very likely is not recommended [1].
APPLICATION OF TITANIUM and its alloys in aerospace, marine, energy chemical process, and other industrial sectors continues to expand. This has been motivated, in part, by trends in decreasing cost and increasing commercial availability of titanium mill products, improved designer and user knowledge and familiarity, and titanium's unique, attractive combination of engineering properties. These properties include, but are not limited to, relatively low density, low modulus, a wide range of strength, and exceptional corrosion resistance over a very wide span of environmental conditions [1,2].
THIS CHAPTER HIGHLIGHTS the corrosion resistance of nickel and its alloys and identifies relevant ASTM standards associated with their evaluation. The test technique selected ultimately will depend upon the specific alloy involved, the type of corrosion in question, or the end application, or a combination thereof. The following discussions will deal primarily with aqueous corrosion and the nickel-base (greater than about 30 % nickel) alloys designed for aqueous corrosion resistance. High-temperature gaseous corrosion will be addressed in other sections.
COPPER AND COPPER ALLOYS are produced in various mill-product forms such as bar, pipe, rod, tube, plate, sheet, wire and as castings. They are used widely because of their excellent corrosion resistance particularly when combined with their other properties, including outstanding electrical and thermal conductivities and ease of fabrication and joining. The copper alloys also possess superior resistance to biofouling.
LEAD IS A MEMBER of Group IVB of the periodic table with two oxidation states (Pb2+ and Pb4+), but the chemistry of the element is dominated by Pb2+ ion. Lead has four isotopes with three of them being the ultimate decay products of uranium (238U and 235U) and, therefore, widely used in geological dating. The crystal structure of lead in solid form is face-centered cubic (FCC) with a lattice parameter of 4.95 at 20C. Lead is a blue-gray metal with density (11.3 g/cm3) 50 % more than that of steel and four times that of aluminum. However, lead is malleable, soft, and melts at only 327C, and therefore, readily cut and shaped into pipes and sheets since ancient times.
ELECTRODEPOSITION OF METALLIC COATINGS has been extensively used as a means of corrosion control. In general, coatings are designed according to one of three different schemes. Electrodeposited coatings may be devised to act as corrosion-resistant barrier layers that separate the substrate from the aggressive environment. Alternatively, the well-known galvanic effects that arise from electrically coupling dissimilar metals may be used to provide active porous coatings, which cathodically or anodically protect the substrate. Electroplating a base metal with a barrier layer of gold is an example of the first strategy. Electrogalvanizing, or depositing zinc on steel, is an example of a sacrificial coating, while thin, porous, noble metal coatings such as palladium or platinum, which catalyze the proton-hydrogen reaction, may be used to anodically protect stainless steels.
THE CHEMICAL PROCESS industry presents a complex set of materials selection challenges. Conducting corrosion testing of candidate materials under simulated or actual service conditions is widely used in the process of materials selection. ASTM G 4, Guide for Conducting Corrosion Tests in Field Applications and G 31, Practice for Laboratory Immersion Corrosion Testing of Metals, and NACE Standard TM-01-69, Laboratory Corrosion Testing of Metals are the general guides for conducting corrosion tests. While these standards can be very useful in making a preliminary list of the best candidate materials, they are designed primarily for the more common metals and alloys such as steel, aluminum alloys, and copper alloys. Certain portions of these procedures, e.g., cleaning methods, are not applicable to zirconium, hafnium, and their alloys. Although ASTM G 2 is designated specifically for zirconium, hafnium, and their alloys, it is a specific practice used in the nuclear industry. Therefore, a tailored practice for conducting corrosion specimen tests on zirconium, hafnium, and their alloys in chemical environments is needed. The use of test standards that have not been modified for zirconium or hafnium may lead to erroneous or invalid results.
A LARGE NUMBER OF tests exist for establishing the reliability of nonmetallic protective coatings on metal substrates. Definitions, fundamentals, methodology, and practical examples are offered for a better understanding of tests for protective coatings and to demonstrate that well defined tests alone are not adequate without rational application and interpretation.
A POWDER METALLURGY part is conventionally made by compacting a lubricated metal powder in a die to the desired shape followed by sintering in a protective atmosphere. Simple and complex parts with densities typically ranging between 80 and 90 % of theoretical and having the finished dimensions are produced at high production rates. The P/M process uses raw materials and energy efficiently and, hence, is cost effective.
FOUR FACTORS SHOULD be considered in selecting corrosion tests for aluminum and aluminum alloys. These are: (1) the expected service environment, (2) the type of corrosion expected in service, (3) the primary material requirements of the application which should not be excessively degraded by corrosion, and (4) whether the material requires surface protection for use in the intended environment. Some form of surface protection is almost always necessary for the high-strength 2xxx and 7xxx series alloys.
STAINLESS STEELS ARE iron-based alloys containing at least 10 % chromium [1]. Although iron may corrode in an ordinary rural environment, the chromium gives stainless steel its ability to form a protective or passive film that resists corrosion. It is this ability to resist the formation of rust that led to the name stainless steel. Generally, it is thought that the passive film consists of hydrated chromium oxide. Other alloy elements such as iron, molybdenum, silicon, et cetera, have also been detected in the passive film. The passive film also tends to incorporate anions and cations that may be present in the environment in which the film forms. Stainless steels are autopassivating in the sense that the passive film is formed spontaneously upon exposure to air, moisture, or an oxidizing acid. When stainless steel is exposed to such an environment, its corrosion resistance approaches that of noble metals; however, when exposed to an environment that damages, reduces, or inhibits the formation of the passive film, corrosion resistance of the alloy is compromised.
THE LARGEST USE of zinc is as a protective coating for steel, and therefore corrosion tests and data [1,2], have been developed and widely used to describe its performance in specific situations or determine its usefulness for new applications. The corrosion resistance of zinc in cast or wrought form is less often a criterion for selection for a given application, although substantial published corrosion data on these product forms exist [2]. Another class of product, zinc anodes, is used to provide corrosion protection to iron and steel structures that are buried or immersed by simply making an electrical connection between the ferrous material and the anode. By this means, it is not necessary to provide a protective coating to the ferrous object. The use of a coating, however, reduces the current demand and extends anode life. Corrosion data for this application are also widely published [1].
COBALT OCCURS IN two atomic forms: a low temperature stable hexagonal close packed (hcp) form and a high temperature stable face centered cubic (fcc) form. The transformation temperature of pure cobalt is 417C. Alloying elements such as nickel, iron, and carbon (within its soluble range) are known as fcc stabilizers, and suppress the transformation temperature. Chromium, molybdenum, and tungsten, on the other hand, are hcp stabilizers and have the opposite effect.
IN MOST INSTANCES, corrosion test methods for plain carbon steels, high-strength low-alloy steels, and alloy steels do not differ greatly. Therefore, these steels are grouped together for the purposes of this chapter. (Alloy steels here refers to heat treatable constructional and automotive steels, and does not include the stainless steels or other high alloys.) There are some differences in the corrosion test methods used for different mill products of this group of steels, and these will be discussed. The steels covered in this chapter are defined below.
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