Choosing Specialty Metals for Corrosion-Sensitive Equipment
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Back to Basics Choosing Specialty Metals for Corrosion- Sensitive Equipment Dean Gambale Tantaline Specialty metals should be considered for processes that push beyond the corrosion limits of steel. New composites and surface alloy technologies expand the options for defending against corrosion. hemical processing facilities are engaged in a including chemical processing, next-generation energy never-ending battle against corrosion to ensure production, oil and gas (onshore and offshore), pharma- Cthat processes operate safely and reliably. Corro- ceuticals, and foods and beverages. sion can affect many types of materials, from metals to Fluoropolymer-coated steel offers both the excellent ceramics, and even polymers and plastics. Many processes corrosion resistance of the polymer (e.g., polytetrafl uoro- employ a variety of materials to protect against corrosion ethylene, PTFE) and the mechanical strength of the steel. and its ensuing problems. These coatings are suitable for many corrosive applica- Some types of corrosion-sensitive equipment, such as tions, but as with any material, they are not universal valves, fi ttings, and instrumentation, require special atten- solutions. For example, polymer coatings may not be ideal, tion. This is because they corrode more quickly than other or even feasible, for instrumentation, because polymers are components in a typical process stream, and even small electrically nonconductive and thermally insulating. amounts of corrosion (about 0.002 in.) can have a large Many polymers are not able to withstand temperatures impact on performance. above 300°F. As the temperature increases, the polymer This article focuses on corrosion-sensitive equipment softens, compromising the mechanical integrity of the typically found in processing plants and the specialty- coating and thus equipment performance. This problem metal material options and techniques that should be is especially relevant under certain conditions, such as considered when stainless steel and fl uorinated polymers a combination of high temperature and pressure, a high are insuffi cient. fl owrate, and/or the presence of mildly abrasive particles. In some applications, the porosity of the polymer can Specialty metals allow corrosive gases to diffuse through the coating and For corrosion-related applications, the term specialty corrode the base steel. In other cases, a polymer lining’s metal generally refers to titanium, nickel alloys, zirco- pores could become contaminated, which can be espe- nium, and tantalum. Specialty metals provide corrosion cially hazardous for pharmaceutical batch processing. resistance and mechanical stability where other materials, In general, process equipment made from specialty such as standard steels, are inadequate. Stainless steels metals is mechanically rugged and provides excellent cor- are mechanically excellent, but their corrosion resistance rosion resistance for many processing applications. Each is limited, especially in concentrated acids at elevated specialty metal has its own niche — a specifi c metal’s temperatures. Although they are usually found in the most strengths and weaknesses may make that material suitable severe applications, specialty metals are attractive for use or unsuitable for a particular processing environment or in corrosion-sensitive equipment in several industries, application. 62 www.aiche.org/cep July 2010 CEP Copyright © 2010 American Institute of Chemical Engineers (AIChE) When selecting materials, consider: involve handling hot concentrated acids (e.g., sulfuric, • estimated service life hydrochloric, and nitric) in polymer production, metal • reliability with respect to safety and economic conse- pickling, acid production, and specialty chemical manu- quences of failure facturing. Due to its negligible corrosion rate, tantalum is • overall availability and delivery time also used in the pharmaceutical and food manufacturing • associated material costs. industries, where even the smallest amount of metallic impurity cannot be tolerated (1). Corrosion resistance Corrosion resistance, which is expressed as the rate Surface alloys, linings, and coatings of corrosion of the metal in the target media, is the most Surface alloys, linings, and coatings were developed to important criterion for evaluating a specialty metal, since combine the best properties of multiple materials. In the this characteristic directly impacts the estimated service case of corrosion protection, the goal is to protect inexpen- life of the equipment. Corrosion resistance data from sive standard steel components with corrosion-resistant laboratory tests are often summarized in an iso-corrosion materials, which are typically more costly. chart, which plots the material’s corrosion rate for a range Coatings are typically applied by dipping, plating or of temperatures and concentrations. Each curve represents spraying metals onto a substrate surface (e.g., carbon a corrosion rate of 5 mil/yr, with points below represent- steel). Various levels of corrosion protection can be ing a lower corrosion rate and points above representing a achieved depending on the materials used. The common higher one. Comparing iso-corrosion curves indicates the feature of coatings is that their bonding mechanisms are relative performance of materials in a particular medium. predominately mechanical in nature. As a result, coatings Although iso-corrosion curves typically depict a cor- rosion rate of 5 mil/yr, it is important to note that a valve 300 corroded at this rate would quickly experience problems, such as leaks and the inability to create tight seals. 250 Figure 1 presents the iso-corrosion curves and relative Zirconium Tantalum corrosion resistances of various specialty metals in sulfu- 200 ric acid (H SO ) and in hydrochloric acid (HCl). In both 2 4 150 acids, tantalum’s corrosion resistance is best, followed by Boiling Point Curve Hastelloy B-3 zirconium, nickel alloys (Hastelloy), and titanium. 100 Niobium Temperature, °C Temperature, Hastelloy C Tantalum 50 Hastelloy B-2 Tantalum is a reactive metal — i.e., it reacts with Titanium 0 oxygen to form a protective oxide layer. What distin- 0102030 40 6050 70 80 90 100 guishes tantalum from other reactive metals is the tenac- H2SO4 Concentration, wt.% ity of its oxide layer and the ease with which it forms. It provides superb corrosion resistance comparable to that 250 of glass. This makes it practically inert to most oxidizing Tantalum and reducing agents, except fuming sulfuric acid, concen- 200 trated hydrofl uoric acid, and hot strong alkalis. Taking no Niobium Zirconium other factors into consideration, tantalum metal is an ideal 150 choice for superb corrosion resistance. However, tantalum metal has some practical limi- Boiling Point Curve tations. First, it is very expensive and typically cost- 100 prohibitive, even when formed around and cladded on top °C Temperature, Hastelloy B-2 of other materials (e.g., steel). Second, the availability of 50 valves, fi ttings, instrumentation, and other process equip- Hastelloy C-22 ment made of tantalum is very limited. Therefore, tanta- 0 0152015 0325 03540 lum (at least in its traditional forms) is only considered for HCl Concentration, wt.% valves, fi ttings and instrumentation in process conditions where no other material could adequately protect against S Figure 1. Tantalum’s corrosion resistance is best for both sulfuric acid the corrosive environment. (top) and hydrochloric acid (bottom), whereas Hastelloy B-2 performs poorly Tantalum is commonly used in applications that according to both iso-corrosion curves. Copyright © 2010 American Institute of Chemical Engineers (AIChE) CEP July 2010 www.aiche.org/cep 63 Back to Basics Surface alloys are an excellent option for ral formation of a dense, stable, self-healing oxide fi lm on its surface. Unalloyed zirconium has excellent resistance applications that require extreme corrosion to sulfuric acid at concentrations up to 60% at the boiling resistance on complex process equipment. point (135°C), as well as resistance to hydrochloric acid at various concentrations below the boiling point. Zirco- can be susceptible to chipping, spalling, or delamination nium is also highly resistant to most alkali solutions, up with mechanical deformation. to their boiling point. Zirconium’s corrosion resistance is Linings such as fl uoropolymers (e.g., perfl uoroalkoxy, comparable to that of titanium in many ways, although it PFA) and metal claddings can also provide corrosion is much more robust than titanium in organic acids, such resistance. Like coatings, linings are held in place mechan- as acetic, citric, and formic acids at various concentra- ically. Although linings are versatile enough to be applied tions and elevated temperatures. Despite these attributes, to both large and small equipment, they are relatively thick, zirconium is vulnerable to corrosive attack by fl uoride usually several millimeters, and held in place mechanically. ions, wet chlorine, aqua regia (nitro-hydrochloric acid), Claddings are formed over a substrate, and in many cases concentrated sulfuric acid (up to 80% concentration), and are considered loose linings because there is little to no ferric and cupric chlorides (3). physical attachment to the substrate. Zirconium is most commonly found in industrial appli- Surface alloys are very different from coatings and lin- cations that produce hydrogen peroxide, manufacture rayon, ings in their fi nal form and effectiveness. Recent develop- and handle phosphoric