Chapter 4: Materials

Materials Upon completion of this chapter, you should be able to recognize the properties of and classify types of: Metals Non-metals Composites Concrete Ceramics The most important materials for most engineering applications are metals, especially carbon steels. This chapter emphasizes the properties of carbon steels, but you can apply the principles associated with these alloys to most other materials used in corrosive environments.

Properties Materials are chosen for a number of reasons, and corrosion resistance is often less important than strength, formability, cost, etc. Metallurgy Fundamentals: Virtually all metals used for engineering applications are alloys. Pure metals are used for electrical conductivity and for some corrosion-resistance applications, but whenever mechanical properties become important, designers choose alloys because they are stronger than pure metals.

Crystal Structure Most solids, with the exception of glasses and organic materials, are crystalline in nature. This means that the atoms in the crystal are oriented in one of seven orientations, only three of which are common in metals. The body-centered cubic (BCC) structure, as shown in Figure below, found in room-temperature iron is stronger and less ductile than the face-centered cubic structure (FCC) that is found in high-temperature iron and in austenitic stainless steels, aluminum, copper, and other ductile metals. This is one reason why steel mills heat steel to high temperatures before many forming operations Zinc and titanium are the only commonly used metals that have the hexagonal-close packed crystal structure not shown in Figure below.

Crystal Structure Body-centered cubic (BCC) structure Face-centered cubic (FCC) structure Grain Boundaries in a Metal

Crystal Structure A solid metal contains many crystals. A typical crystal has millions of atoms, and defects in the structure are common. The combination of alloying additions, crystal size, and different crystal structures determines the mechanical and corrosion resistance properties of metals. The grain boundaries, indicated by the dotted lines in Figure above, have larger spaces between the atoms. Impurity atoms are most likely to be in these spaces. Unwanted segregation of impurity atoms to grain boundaries can cause major problems with corrosion resistance.

Crystal Structure Most metals are alloys. The effects of alloying on the metal microstructure can result in: Atoms of similar size to the base metal fitting into the base-metal crystal structure, forming substitutional solid solutions as shown in Figure below. Small atoms fitting between the larger atoms forming interstitial solid solutions as shown in Figure below. Carbon, oxygen, boron, and nitrogen may be used for interstitial case hardening of steel. Hydrogen and helium are even smaller—too small to be controlled—and they can cause hydrogen or helium embrittlement. New crystals, having different compositions and crystal structures than the base metal. As an example, carbon steel usually has almost pure iron crystals plus a compound having the chemical formula Fe3C, this iron carbide phase gives steel greatly increased strength compared to pure iron. Two-phased metals are usually stronger than single-phased metals.

Crystal Structure

Strengthening Methods Metals are strengthened by: Alloying—All metals used for strength are alloys of two or more different elements. Precipitation hardening—The metal is heat treated to produce crystals that would not form without special heat treatment. Work hardening—The metals are deformed at low temperatures producing hardening due to deformation. Grain size refinement—Metals with small crystals are stronger than metals with larger crystals. Second-phase hardening—Steel is a good example of this.

Mechanical Properties Metals are chosen for their strength and related mechanical properties, including: Tensile and yield strength Toughness Ductility Fracture Hardness Stress concentrations Strength Most metals are specified based on their strength. Most industrial specifications define strength targets for yield. Hardness is a material property that may be important for materials that will have wearing surfaces. Hardness testing is also used for field checking of mechanical strength, because tensile strength and hardness can be correlated.

Mechanical Properties Ductility Ductility is the ability of a metal to be stretched in tension before it fractures. In addition to elongation before breaking, some definitions of ductility specify the reduction of cross section area before breaking as a measure of ductility. The opposite of ductility is brittleness. Some authorities consider any metal to be brittle if it has less than 5% elongation before breaking. Toughness Toughness is a measure of the resistance of a material to a propagating crack during impact loading. Toughness, as property, has gained importance in recent years, and has been added to many materials specifications.

Mechanical Properties Fracture The forms of fracture for many metals are: Overload (ductile) fracture Brittle fracture Creep Fatigue Stress Concentrations Modern engineering designs have adopted the practice of avoiding sharp details (e.g. 90 degree angles with no rounding) on hatch covers and other procedures to limit the effects of stress concentrations. Stress concentrators raise the effective stress on a part and can cause brittle failures. Examples of stress concentrators due to corrosion include stress corrosion cracks and corrosion pits.

Forming Methods Forming Methods Just like wood has different strength properties in different directions, metals reflect their forming properties. This results in different mechanical and corrosion resistance properties depending on how a metal is formed. Wrought vs. Cast Structures Wrought is defined as material structure altered by thermo-mechanical processes, e.g. rolling, extrusion, forging, drawing. Typically small grain size Cast is defined as material structure achieved by solidification from the molten metal. Typically large grain size Cast metals typically have larger crystals and are weaker than wrought products.

Forming Methods All wrought (deformed after solidification) metals have crystal structures that reflect their forming process. Rolled plate, which is used to manufacture large diameter pipe and pressure vessels, will have different microstructures and different mechanical properties in all three principal directions. The increased number of exposed grain boundaries on the short-transverse direction usually leads to lower corrosion resistance than for the other directions. Castings tend to be weaker than wrought because their crystals are larger. They may also have porosity that would be eliminated by rolling or other deformation processes in wrought products. Typically, applications where complex shapes are more economical use castings instead of forming, machining, or welding wrought products into the desired shape. Many castings have thick cross sections that make them less subject to failure by localized corrosion, e.g. pitting corrosion.

Forming Methods

Welding Welding is the preferred joining method for many structures; especially those intended for immersion or buried service. Problems associated with welds that may affect their corrosion resistance include: Heat-affected zones (HAZs)—The microstructures in HAZs can produce phase changes that may lead to HAZ corrosion. HAZs can also be harder than the weld metal or base metal, thus making it more prone to hydrogen embrittlement if not post-weld heat-treated. Porosity —This may provide stress concentrations. Cold cracking—Hydrogen cracking is the principle cause of this problem. Hot cracking—Also called sulfur cracking. Lack of fusion—Sometimes the base metal does not melt, and the weld bead and the base metal do not join together in a continuous metal structure. Incomplete penetration—The weld bead does not go deep enough into the surface of the parent metal. Striking marks—These can cause hard spots.

Welding

Materials Specifications Many organizations provide specifications for materials, and as a best practice you should order materials based on standardized materials specifications. Metals Carbon Steel Carbon steel is the most commonly used metal in industry. Yield strength from about 250 MPa (36 ksi) to over 1380 MPa (200 ksi). These materials normally contain approximately 0.2% carbon, although this varies widely. Low-Alloy Steels A common definition of low-alloy steel is steels with alloying contents of 8% or 9%. They are usually specified for their improved mechanical properties, e.g. in pressure-vessels, boiler, and other high-temperature applications. They are usually cathodic to carbon steels, but, like carbon steels, they normally have insufficient corrosion resistance.

Materials Specifications Weathering Steels Weathering steels contain trace amounts of copper and can develop protective corrosion product films in certain atmospheric environments. They are typically not painted and are used when no painting is required for atmospheric exposure. Cast Irons Cast irons are alloys of iron and carbon having approximately ten times the carbon content of typical carbon steels. The carbon is usually in the form of graphite flakes (gray iron) or spherical nodules (ductile iron) Cast iron classifications include: Gray cast iron White cast iron has less silicon than grey cast iron, Nodular cast iron is often called ductile cast iron, Malleable cast iron is produced by heat-treating white cast iron, High silicon (typically 14% Si) is brittle and used extensively for impressed current cathodic protection anodes

Materials Specifications

Materials Specifications Stainless Steels Stainless steels are the most commonly used corrosion resistant alloys (CRAs). NACE defines CRAs as alloys whose mass loss rate in produced fluids is at least an order of magnitude less than that of carbon and low alloy steel. Stainless steels are normally classified based on their microstructure, and usually are defined as iron-based alloys having at least 11% chromium by weight. The major classes of stainless steels are: martensitic, ferritic, austenitic (and superaustenitic), duplex (and superduplex), and precipitation hardening.

Materials Specifications The corrosion resistance of stainless steels depends on many factors which include alloy composition and environmental parameters such as chloride content, pH, temperature, etc. The Pitting Resistance Equivalent Number (PREN) is sometimes used to provide a qualitative indication of corrosion resistance of stainless steels. It is based on an empirical formula such as: PREN = wt% Cr + 3.3 x wt% Mo + 16 x wt% N A higher PREN value generally indicates greater resistance. However, there is no threshold PREN which indicates complete corrosion resistance in all environments.

Materials Specifications Martensitic Chromium (11%–17%), Carbon vary from 0.10%–0.65%. The high carbon enables the material to be hardened by heating to a high temperature, followed by rapid cooling (quenching). Martensitic types offer a good combination of moderate corrosion resistance and superior mechanical properties, as produced by heat treatment to develop maximum hardness, strength, and resistance to abrasion and erosion. End uses include cutlery, scissors, surgical instruments, wear plates, garbage disposal shredder lugs, industrial knives, and steam turbine blades. Martensitic stainless steels are magnetic. These alloys require protective coatings and cathodic protection when used for pipelines—they are not adequately corrosion resistant for direct burial or long-term immersion service. Cathodic over-protection can lead to hydrogen embrittlement.

Materials Specifications Ferritic Ferritic stainless steels have higher chromium levels than martensitic stainless steels. Ferritic stainless steels are magnetic, generally have good ductility, and can be welded and/or fabricated without difficulty. These grades can be processed to develop an aesthetically pleasing, bright finish, so they are sometimes used for automotive trim and appliance molding. Functional applications in which cost is a major factor, such as automotive exhaust systems, catalytic converters, radiator caps, and chimney liners use ferritic stainless steel. Ferritic stainless steel can be hardened by cold rolling, but cannot be hardened as much as the austenitic alloys.

Materials Specifications Austenitic and Super-Austenitic The majority of stainless steel used worldwide is austenitic stainless steels. These alloys are ductile because of their FCC structure, which also means they cannot be heat-treated for strength. Austenitic stainless steel includes both the 200- and 300-series alloys, which can be hardened by cold working. The 300-series alloys contain chromium and nickel as their major alloying additions. Type 304, also known as 18-8, is the most widely used of all stainless steel alloys. The 304 stainless is the “standard” austenitic stainless steel. Type 316 has Mo additions to limit pitting and crevice corrosion resistance.

Materials Specifications Austenitic and Super-Austenitic Austenitic stainless steels are subject to sensitization— the formation of unwanted chromium carbides in grain boundaries leading to accelerated corrosion due to chromium depletion in grain boundaries. Sensitization can be caused by the heat input of welding, for example in heat- affected zones (HAZs), or by long-term exposure to elevated temperature. Limiting the carbon content of the alloy can minimize sensitization due to welding, but is not effective for mitigating sensitization when caused by long- term elevated temperature exposure. Types 304L and 316L stainless steel have lower maximum allowable carbon contents than 304 and 316 (0.03 C vs. 0.08 C max). Type 317 SS has a higher Mo content requirement than 316, this is predicted to produce greater pitting and crevice corrosion resistance. The 200 series alloys possess mechanical and corrosion resisting properties similar to 300 series materials. They also exhibit high hardness and yield strength as well as excellent ductility and are usually non-magnetic.

Materials Specifications Super-Austenitic Highly-alloyed FCC alloys often are called super-austenitic stainless steels. Pure nickel is FCC, so the crystal structure of these alloys is FCC if iron or nickel is the predominant component. All of these alloys are called highly-alloyed austenitic stainless steels in NACE MR0175/ISO 15156.8 Many of these alloys contain less than 50% iron. They contain 4–6% molybdenum to improve resistance to chloride environments. They are about 50% stronger than austenitic stainless steels.

Materials Specifications Precipitation-Hardening Precipitation-hardening (PH) stainless steels are based on the general composition of 17 Cr-4 Ni, and this nickel content is lower than the 18-8 austenitic alloys. The alloys can be machined in the soft state, and then heat treated to improved mechanical properties. These materials can be hardened to a strength up to 200 ksi (1350 MPa). Austenitic stainless steels, which are intended to be single-phased FCC metals cannot be heat treated for this purpose.

Materials Specifications Duplex and Super Duplex Duplex stainless steels have a combination of ferrite and austenite (approximately 50/50) in their microstructure. Ferrite is the continuous phase while austenite is the discontinuous phase. Duplex stainless steels have similar pitting resistance to austenitic stainless steels, but their chloride stress corrosion-cracking resistance is superior due to ferrite being the continuous phase. (Austenite is susceptible to chloride SCC, and ferrite is susceptible to H2S.) These materials are about twice as strong as commonly-used austenitic stainless steels.

Materials Specifications Copper Alloys Copper alloys have been used for many years and were probably the first CRAs before the Iron Age. Bronze Age tools, made from naturally-occurring deposits of copper mixed with tin, had advantages over stone tools, because they were less brittle and could be sharpened. They were harder than copper, gold, and silver—metals that were discovered before bronze. Tin is a relatively expensive metal, and modern bronze alloys will often be copper-aluminum alloys instead of copper-tin alloys. Most copper-based alloys are relatively low strength compared to other alloy systems.  

Materials Specifications Characteristics of Copper Alloys The corrosion resistance of copper alloys is primarily due to the noble position of copper in the EMF series. The presence of oxidizers, such as nitric acid or oxygen, can cause copper to corrode in aqueous solutions. In some aqueous solutions, a passive oxide film is formed on copper alloys. If stable, this film can reduce corrosion. However, the film can be removed by high velocity flow; so copper alloys are generally subject to erosion-corrosion in high-velocity flow conditions. Copper alloys can also be attacked in aqueous solution by carbon dioxide, acids, chlorides, sulfides, and ammonia. In mild atmospheric environments, many copper alloys become coated with a thin protective layer of corrosion products composed of cuprous oxide (Cu2O) and called a patina. Copper alloys are not, in general, resistant to atmospheric environments containing ammonia.

Materials Specifications Characteristics of Copper Alloys Copper alloys are widely used for water piping, pumps, valves, heat- exchanger tubes and tube sheets, hardware, wire, screens, shafts, roofing, tanks, and vessels. Many copper alloys are resistant to seawater fouling and are used in seawater service. Copper-zinc alloys are called brasses and contain from 10%–45% zinc. The addition of zinc increases susceptibility to stress corrosion cracking. Aluminum bronzes, more commonly nickel aluminum bronzes, have become the preferred casting alloys for large pumps, valves, and similar applications in seawater.

Materials Specifications Titanium Alloys Unalloyed titanium is essentially inert in ambient-temperature seawater service. The palladium-added alloys were developed for increased crevice corrosion resistance at elevated temperatures. The aerospace industry uses titanium alloys because of their excellent strength-to-weight characteristics. Other alloys are used for their corrosion resistance as shown in Figure below. Titanium alloys have excellent corrosion resistance, but their heat transfer properties are not as good as copper and stainless steels.

Materials Specifications While titanium alloys generally have good corrosion resistance, there are several potential issues with their use. Titanium is cathodic to most other alloys, which means that titanium piping systems should not be mixed with other alloys. Unfortunately, titanium can suffer hydrogen embrittlement and stress corrosion cracking (SCC) at elevated temperatures in chloride environments. Titanium alloys are also difficult to weld and require special cleaning procedures to ensure oxide-free surfaces prior to welding. While titanium has excellent corrosion resistance in many environments, the passive film on titanium requires the presence of environmental oxygen, and exposures in reducing environments lead to very rapid corrosion.

Materials Specifications Aluminum Aluminum and aluminum alloys are very widely used where weight is a significant factor. Aluminum is a reactive metal, but is passive in many mildly corrosive environments because of the formation of a thin transparent film of aluminum oxide. In more corrosive environments, this film can break down and corrosion can occur. The film is generally stable in neutral and oxidizing acidic environments, but is generally unstable in reducing acids, alkaline environments, and environments containing chlorides. The oxide film on aluminum alloys can be artificially thickened through a chemical treatment called anodizing.

Materials Specifications Aluminum Aluminum is sensitive to small concentrations of heavy metals. The presence of heavy metals increases the aluminum corrosion rate substantially. Aluminum cathodic protection anodes have intentional additions of heavy metals as alloying elements to prevent passivation of the alloy. Aluminum alloys are anodic to most engineering metals. Aluminum alloys are very easy to form and fabricate and can be welded. One factor in the use of aluminum alloys is that their corrosion products are nontoxic and are colorless or white.

Materials Specifications Aluminum Aluminum is a very reactive metal, and the natural oxide film on aluminum surfaces is normally much thicker than on other CRAs. The metal does not need to be painted in the marine atmosphere. Aluminum is also ductile at cryogenic temperatures, this makes aluminum alloys suitable for cryogenic piping and storage tanks. Different aluminum alloys, as well as special alloys of zinc and magnesium are used as sacrificial anodes for cathodic protection, and these alloys are discussed in Chapter 6.

Materials Specifications Zinc Coatings are the largest user of zinc. Paints use zinc-rich pigments, and thin coatings on metal surfaces use hot-dipped and electroplated zinc. Zinc, like aluminum, is an amphoteric metal.