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Home , Corrosion, Rust, Sacrificial metal

1 AND CORROSION- PROTECTION

We begin our study of engineering materials with the subject of corrosion and how we can protect against it. Corrosion is one of the principal ways in which metallic materials fail, and it is one of the best understood. Moreover, it is commonly treated in introductory courses in chemistry, so we can start here by building on the experience of most students. In addition, an understanding of the elements of corrosion at this level is of immediate applicability to some problems of everyday life. We use the bicycle wheel to illustrate one such application.

1.1 Corrosion of Corrosion involves the ionization of atoms and the loss of these into solution or into a corrosion product. Since the ionization reaction me~ns giv­ ing up electrons, a flow of electrons away from the site of this reaction must occur to avoid a build-up of negative charge. Thus, corrosion is an electrochem­ ical reaction. The site where the loss of metal occms is called the , or anodic region, and the ~lectron s flow through the metal to a site, called a , where they are consumed in a cathodic reaction. In the case of , the anodic reaction is usually Fe --;;. Fe++ + 2e- and the cathodic reaction, in the presence of and sufficient , is usually

H20 + 112 0 2 +2e- --;;. 2 OH- The corrosion product, , forms from Fe++ + 2 OH- --;;. Fe(OHh The actual electrochemical mechanism can be appreciated if one considers how a rust forms. A pit begins at some inhomogeneity on the surface, such as 8 an impurity particle, and the above reactions occur. The pit-type geometry forms because the anodic reaction continues to occur underneath the rust cover, as 8 shown in Fig. 1.1.

Air

Steel Fig. 1.1 Corrosion occurs under the rust. where the oxygen content is lower. The result is the formation of a pit.

1 CHAPTER I CORROSION AND CORROSION PROTECTION

It is useful to consider the formation of a rust pit in some detail, because it helps one understand the electrochemical nature of corrosion more clearly. The important questions are: where is the anode and where is the cathode? Since the cathodic reaction employs water and oxygen to use up electrons, it must occur where the water and oxygen are available. This locates the cathode at the swface of the at the periphery of the rust. The anode is then at a location where water and oxygen are less concentrated, which is underneath the rust. Thus, iron is ionized under the rust, and the electrons flow to the surface alongside the rust, to be consumed in the cathodic reaction. Hence, material loss proceeds under­ neath the rust, and the result is a pit.

1.2 Corrosion Protection of Steel can be protected from corrosion by it with a more reac­ tive "sacrificial" metal. If the coating is , the product is referred to as galva­ nized steel; it can be made, for example, by dipping the steel into a bath of molten zinc. A is one that undergoes the anodic reaction in preference to another, more "noble" metal. That is, the sacrificial metal has a greater ten­ dency to lose electrons. This tendency can most usefully be expressed in what is called a "." Table 1.1 gives such a series for some and alloys in sea water. This table gives only relative positions, rather than quantita­ tive differences, because it is not based upon a standardized testing condition.

Table 1.1 Galvanic Series in Sea Water Cathodic - noble 18-8 (Cu-Zn) Nickel - Iron or carbon steel Aluminum Zinc Anodic - active

The meaning of Table 1.1 is that zinc or cadmium, being more active than iron or carbon steel in this series, will, if electrically connected to iron, act as the anode, and the electrons released will then flow to the iron, which will be forced to serve as the cathode, as shown in Fig. 1.2. The cathode does not dissolve, so the iron remains intact as long as any of the more active metal remains close by. Environment

Fig 1.2 The release of electrons by the more active zinc layer causes Carbon Steel (mostly iron) "------...- the iron to be cathodic. 2 SECTION 1.2 CORROSION PROTECTION OF STEEL

One might think to protect the carbon steel by , which gives a brighter surface than does zinc. However, the chromium plating is not sacrificial. Chromium plating is actually mostly nickel (plated over a thin layer of copper, used to make a better bond with the steel), as shown in Fig. 1.3. The relatively thick nickel layer is then covered by a very thin layer of chromium to keep the surface bright.

Environment

------Cu Fig. 1.3 If the coating on a chromium-plat­ Carbon Steel ed steel is breached, the exposed carbon steel can become a very active anode, and a pit can form.

If this coating is breached locally (e.g., by mechanical damage), the iron is exposed in a small area that is surrounded mainly by nickel. Since nickel is noble with respect to iron (Table 1.1 ), it acts as a cathode and the iron as an anode; therefore, a rust pit forms. It tends to form quickly, since the cathode:anode area ratio is large; this gives a high current density at the anode. The electrons that leave the anode area represent the corrosion current In the case where iron atoms become ions, two electrons leave for every created. One could calculate the weight loss by measuring the corrosion current and knowing the atomic weight of iron. For example, a current of about 3xl015 electrons would remove one atom layer from an anode area of lcm2. However, if the anode area were confined to lrnm2, the same current would make a pit 100 atom layers deep. Thus, for a given area of cathode, the smaller the anode area is, the deeper the pit. A rust pit is undesirable for cosmetic reasons, but it can also serve as a stress concentrator, leading to early failure, for example. Chromium resists corrosion by a process called . Passivation involves the rapid formation of a thin layer by the chemical combination of oxygen atoms with surface atoms of the metal; this isolates the metal from the environment and prevents further oxidation. The oxide, which is so thin that it is transparent, must be highly stable, well-bonded to the metal, and free of pores and other defects. Once formed, it must have an extremely slow rate of thicken­ e ing. With chromium, the protecting oxide is Cr 0 . 2 3 Passivation is also responsible for the corrosion resistance of metals like alu­ minum and titanium, which also form very stable . That is, the driving force to form the oxide is large. Once formed, the oxide is very difficult to reduce. (That is why these metals are relatively expensive to produce; it requires large amounts of energy to reduce the oxide ores to produce the free metal.) A solid piece of chromium would retain its passivated character even if the surface oxide were scratched away. This is because there is always more of the chromium below to form oxide and re-establish the protection. Only factors that inhibit the oxide formation would cause problems. ions act this way, so

3 CHAPTER 1 CORROSION AND CORROSION PROTECTION

water environments are dangerous to metals protected by passivating oxides. Stainless steel works on the same principle as a piece of solid chromium. e There are enough chromium atoms dispersed in the crystal lattice of stainless llli/' llllllll/!11 FeCr204 steel to provide an oxide sufficiently rich in chromium to passivate the surface. Fe, Cr In contrast to stainless steel, the problem with so-called chromium plating is that, once the underlying carbon steel is exposed, a protective oxide does notre­ form over it; therefore, any defect in the plating will ultimately lead to a tiny rust pit. (The rust can be cleaned off by abrasion, but the reaction will continue, and more rust will eventually erupt from the pit. The process can be retarded some­ what by applying a coating, such as a wax, which tends to seal off the pits.) Clearly, stainless steel is far superior to chromium-plated carbon steel.

1.3 Constitution of Stainless Steel In stainless steel, which is usually more than 70% iron, the chromium exists in solid solution in the iron, meaning that the chromium atoms simply substitute for iron. atoms in the crystal structure, as indicated in Fig. 1.4. This is called a substitutional solid solution. The solid solubility can be large when the solute atom is nearly the same size as the host atom and not too different chemically. Another kind of solid solution occurs with solute atoms that are very much smaller than the host atom and are thus able to fit into the interstices between the host atoms in its crystal (cf. Fig. 1.4). This is called an interstitial solid solution. The classical example is carbon in iron, which will be discussed in great detail later. A solid solution is an example of a solid phase. A phase can be defined as a homogeneous body of matter having a distinct structure (i.e., atomic arrange­ ment) and that can, at least in principle, be mechanically separated from a sur­ rounding phase (or phases).

interstitial solute atom

Fig. 1.4 Stainless steel is a solid solution, having both sub­ stitutional solutes (Cr and Ni) and interstitial solutes (C and substitutional N). The latter are usually treated as impurities in stainless solute atom steels.

Example 1.1 : Q. How many phases are present in a of ice water? A. Three: liquid, solid, and vapor. Q. Is the number of phases changed if something is dissolved in the water? A. No, but the water becomes a liquid solution, and the ice would become a solid solution, if the substance dissolves in it.

4 SECTION 1.3 CONSTITUTION OF STAIN LESS STEEL

It has been found empirically that at least 12 wt% chromium in solid solution is necessary to make stainless steel. However, it turns out that an Fe-12 % Cr alloy tends to have low ductility and toughness, so it would not be easy to make a wire spoke out of it. Ductility refers to the ability of a piece of metal to be stretched (permanently) by plastic deformation. (It is related to malleability, the ability to be processed (by rolling) into thin sheets.) Toughness refers to the resistance of a material to the propagation of a crack; it is the opposite of . The low ductility and toughness of Fe-Cr alloys at ordinary is related to their crystal structure, which is the same as in pure iron: body-centered cubic, or BCC, as shown in Fig. 1.5. To get around the low-ductility problem, it was found possible to change the crystal structure of an Fe-Cr alloy by adding a sufficient amount of nickel. Alloys will be discussed later, but that is why the nickel is present; it does not play an important role in the corrosion protection. e The most common stainless steel is the " 18-8" austenitic stainless steel, mean­ ing it contains 18 wt% chromium and 8 wt% nickel and has the face-centered cubic, or FCC, crystal structure of the phase called , named in honor of W. C. Roberts-Austen, an early pioneer in the study of steels. It is illustrated in Fig. 1.6. One attractive feature of this crystal structure is that it is almost always ductile and can be readily cold-drawn into wire, ideal for making bicycle spokes, for example.

(a) (b)

Fig. 1.5. The BCC unit cell, expressed as (a) a ball-and-stick model, showing the locations of the atom centers, and (b) a hard-sphere model, showing that atoms touch along the diag­ onals of the cubic unit cell (i.e., the body diagonals).

(a) (b)

Fig. 1.6 The FCC unit cell. The atoms touch along each of the face diagonals.

5 C H APTER 1 CORROSION AND CORROSION PROT ECTION

Example 1.2: a. In a cubic crystal (e.g., BCC or FCC) the lattice parameter a is the length of an edge of the unit cell. Calculate the length of the diagonal of a face and of the cube (the body diagonal). A. Face diagonal: ..Va2 + a2 = a-v2

Body diagonal: ..Va2 + a2 + a2 = a{3

Example 1.3 a. Calculate the packing density of a BCC crystal; i.e., the percentage of the unit cell occupied by atoms, assuming the hard-sphere model. A. In a BCC unit cell the atoms touch along the body diagonal. Let the atomic radius be r. Then 4r = -13a , and the volume of the unit cell is 3 a 3 = 64r 3..f3 There are two atoms per unit cell: 1/8 per corner x 8 corners = 1, plus the center atom = 2 Therefore, the total volume of atoms is 2 x 4/3 n ~ . and 3 the packing density = volume of atoms = 8/3 .Ji = 0_68 volume of unit cell 64/3 3 r3 Thus, 68% of the BCC unit cell is occupied by solid matter, according to the hard-sphere model.

Example 1.4 a. In an 18-8 stainless steel, what is the atomic % chromium? A. 18-8 stainless steel contains 18 wt% chromium and 8 wt% nickel. Approximate atomic weights (from the , inside back cover): Fe 56 Cr 52 Ni 59 Consider 100g of the stainless steel; it has 18g Cr, 8g Ni, and 74g Fe. A . t t . o/c C - 18/52 pprox1ma e a om1c o . r - + + 19 7 4156 18152 8159

6

------SECTION 1.4 THE BICYCLE WHEEL

1.4 Case Study: The Bicycle Wheel The modern bicycle wheel functions in a fundamentally different way from the traditional wagon wheel. As depicted in Fig. 1.7(a), the load on a wagon wheel is supported by the compression of the bottom spoke, with the rim trans­ mitting lesser compressive loads to the adjacent spokes. The spokes in the upper part of the wheel carry none of the load. The spokes must be thick enough to resist failure by buckling, which occurs when the compressive load on a thin rod, or column, reaches a critical value such that even a slight deviation from pure axial alignment of the load causes instability and collapse of the column. In comparing the bicycle wheel to the wagon wheel, an illustrative analogy would be that the former is to the latter as a suspension is to a cathedral. The wagon wheel and the cathedral are both restricted to compressive loading of their components, whereas the suspension bridge and the bicycle wheel both employ stretched wires to support their loads. The analogy is imperfect, however, in that the loaded hub of the bicycle wheel is not simply suspended from the upper spokes, like a roadway is hung from its suspension cables. Rather, the spokes in a bicycle wheel are pre-tensioned during construction of the wheel, like the strings on a guitar.

(b) t

Fig. 1.7 (a) A wagon wheel, showing that the load is carried by the compression of the bottom spoke. (b) A modern bicycle wheel. (From J. Brandt, The Bicycle Wheel, Avocet, Menlo Park, CA, 1981, pp. 11 and 12.)

Figure 1.8 illustrates the geometry of a conventional spoke and a nipple and the method of their attachment to the hub and rim of a wheel. The tensioning of each spoke is carried out by turning the nipple with a spoke wrench. When a wheel is properly tensioned, the rim is loaded in uniform circumferential com­ pression, such that it is not distorted out of its plane. In this case the wheel is said to be "true." In contrast to the wagon wheel, when a bicycle wheel is loaded, the tension in the bottom few spokes is decreased significantly, and the tension in all the rest of the spokes is increased by a small amount. The important point is that all the

7 CHAPTER I CORROSION AND CORROSION PROTECTION

spokes remain stressed in tension; no spoke is ever in compression. In fact, it is physically impossible for a spoke to be loaded in compression, due to the method of attachment to the rim (cf. Fig. 1.8). This is essential, of course, because the spokes are thin wires, and the critical load for buckling would be very small.

Fig. 1.8 Schematic illustration of a spoke and a nipple and of the method of their attachment to the hub and rim of a wheel. {Adapted from J. Brandt, loc. cit.)

The tensile stress in each spoke is considerable. A typical tensioning load is 90 kg, and a typical spoke diameter is 1.6 mm. This gives a tensile stress of about 440 MPa, which is around one-half the value needed for permanent deformation (i.e., the elastic limit). (The primary strengthening mechanism for spokes is the subject of Chapter 4.) It must also be recognized that the stress on a given spoke in a rotating wheel varies with time and also varies with the actions of the rider and the conditions of the road surface. For example, if the rider is coasting (i.e., not pedaling) on a smooth surface, a given spoke would be partially unloaded every time this spoke became the bottom spoke. The effect of a pot-hole in the road would be to accen­ tuate this unloading. This cyclic behavior is illustrated schematically in Fig. 1.9. Cyclic loading can lead to failure by fatigue.

pre-tension of spokes in unloaded ---+­ wheel Stress in One Spoke one revolution 0 of wheel roadshock ~------~~~------~~~------Time Fig. 1.9 Schematic illustration of the variation of the stress on a particular spoke with time as a wheel rotates, including the effect of a pot-hole in the road surface. (After C.J. Burgoyne and R. Dilmaghanian, J. Eng. Mech., ASCE, Vol. 11 9, March 1993, p. 439.)

Fatigue is a process of gradual of a component subjected to a cyclic, or repeated, load. In this process a fine crack nucleates at the surface (usually) and grows to a size that produces sudden failure. (This is what happens when one I breaks a s~ort piece of wire from a longer piece by repeated bending back and I forth.) The mechanism of fatigue failure and how it can be resisted are dealt with in Chapter 6. Obviously, as a bicycle wheel turns, the spokes go through cycles of greater and lesser tension (cf. Fig. 1.9); hence, they are subject to fatigue failure. Such a failure in the front wheel can cause the rider to be pitched over the han­ dlebars, if the broken spoke falls across the front fork and causes the wheel to jam.

8 CHAPTER I GLOSSARY/VOCABULARY

The choice of carbon vs. stainless steel spokes hinges on which factor is con­ sidered more important: resistance to corrosion or resistance to fatigue failure. The available means of corrosion protection for carbon steel are only transitory, as we have seen. However, carbon steel has an advantage over stainless steel in that it is more resistant to the other common mode of spoke failure, fatigue. Cyclists who ride thousands of miles per year are more concerned about fatigue failure than about corrosion, so they sometimes choose carbon steel spokes. They re-lace their wheels with new spokes so frequently that there is not enough time for significant corrosion to develop in the carbon steel spokes (which are temporarily protected by of cadmium or zinc).

Summary

Corrosion involves loss of metal atoms at an anode and consumption of the resulting electrons at a cathode. Carbon-steel spokes are temporarily protected from corrosion by coatings of cadmium or zinc, which act as sacrificial . Stainless steel spokes are self-protecting by the formation of a passive surface layer comprising a very thin mixed-Fe-Cr oxide. This requires the presence of chromium atoms in solid solution in the iron; a minimum of 12wt% chromium in solid solution is required for this protection; bicycle spokes normally contain about 18wt%. About 8wt% nickel is also added (making "18-8" stainless steel, which is the most common variety) for the purpose of converting the crystal structure from BCC to FCC. The latter is much more amenable to mechanical processing by wire drawing. The permanent corrosion resistance of the stainless steel makes it preferable to the more fatigue-resistant carbon steel for most riders.

Glossary/Vocabulary - Chapter 1

Anodic reaction In corrosion, the reaction at the anode involves the conversion of metal atoms to ions, with the concomitant release of electrons. Examples: Fe ---> Fe++ + 2e· and Zn ---> Zn++ + 2e· Body-centered This is a crystal structure with a cubic unit cell and with a lattice site on each cubic, BCC corner and one in the center of the cube. Iron is BCC at temperatures up to 910°C. Chromium and are BCC at all temperatures up to the melt- ing point. Cathodic reaction In corrosion, the reaction at the cathode is one in which electrons are consumed. Example: H20 + 1/2 02 + 2e· ---> 20H- Coordination This is the number of atoms that surround a given atom in a crystal, or the aver- number age number in a disordered solid or a liquid. The coordination number in a BCC crystal is eight. In an FCC or HCP crystal it is twelve. Cyclic loading The repeated loading of a specimen or component is called cyclic loading. Bicycle spokes are cyclically loaded, going from high tension to lower tension once for each revolution of the wheel.

9 CHAPTER I CORROSION AND CORROSION PROTECTION

Face-centered This is a crystal structure with a lattice site at each corner of the cubic unit cell cubic, FCC and one in the center of each face. Iron is FCC at temperatures between 910 and 1394°C. Aluminum, gold, silver, nickel, and copper are all FCC. Galvanic series A listing of metals and alloys in order of their tendency to be active (anodic) vs. noble (cathodic) in a particular environment, such as sea water. Zinc and cadmi­ um are active with respect to iron in the galvanic series in sea water. Hard-sphere model A model of a structure, like a crystal structure, in which atoms are depicted as hard spheres, as opposed to a ball-and-stick model. The hard-sphere model is appropriate for ionic crystals and for metallic crystals in which directional bond­ ing is negligible. The ball and stick model is more appropriate for covalently bonded crystals or molecules. Hard-sphere models are normally used for FCC, BCC, and HCP crystals. Noble vs. active A is one from which it is difficult to remove electrons. Thus, a noble metal resists the anodic reaction. The opposite is an active metal. Metals at the top of the Galvanic series, like gold and platinum, are noble. Packing density The fraction of the volume of a unit cell that is occupied by atoms (assuming a hard-sphere model) is called the packing density. The packing density of a BCC unit cell is 0.68. Passivated surface The formation of a continuous (i.e., complete) surface layer of oxide, which iso­ lates a metal from an aggressive environment and is self-healing when disrupt­ ed, is said to passivate the surface. That is, it converts an initially active metal into a passive state. The oxide layer must remain very thin; i.e., the kinetics of

thickening must be very slow. Aluminum is passivated by formation of Al20 3. Titanium is passivated by formation of Ti02. Chromium is passivated by Cr20 3 Phase A phase is a physically distinct body of matter that is, in principle, mechanical­ ly separable from surrounding matter. It may be gaseous, liquid, or solid. In a mixture of solid phases, each has a particular structure (i.e., atomic arrangement) and composition. Ice in water is a two-phase mixture. (Three, if we count the vapor phase above the mixture.) Austenitic stainless steel is a single-phase alloy; a solid solution is a phase. Pitting is the result of localization of corrosion. That is, the anode remains local­ ized and is surrounded by a cathodic region. The opposite case is general corro­ sion, wherein anodic and cathodic regions are distributed more or less u_niform­ ly over a surface. Pitting can occur under the rust formed over a local anodic region, because the oxygen concentration is higher alongside the rust, making the latter regions cathodic. Sacrificial anode A relatively active metal when connected electrically to a less-active metal acts as a sacrificial anode, because it suffers corrosion in preference to the less-active metal. This occurs as long as the electrons so produced can flow to the less~active metal, which serves as the cathode. Examples: Zinc plating or cadmium plating on carbon steel.

10 CHAPTER I E XERC ISES

Solid solution A crystalline solid, like a metal, may contain foreign atoms within its crystal structure, either substitutionally or interstitially. This constitutes a solid solution. The foreign atom (either an impurity or an alloying element) is the solute, and the host element is the solvent. Chromium and nickel are substitutional solute elements in austenitic stainless steel. Carbon is an interstitial solute in steel. That is, it is too small to substitute for an iron atom on the crystal lattice, so it resides in an interstice (the space left by a group of touching atoms). Stainless steel Any steel that contains a minimum of 12% chromium in solid solution is con­ sidered "stainless" in that it does not rust when exposed to water and oxygen. The reason is that the surface is passivated by the formation of a mixed Fe-Cr oxide. Normally, stainless steel contains at least 18%Cr to allow for the precip­ itation of some of the chromium by carbon. (Cr is a strong carbide-former.) Fe+18%Cr would make a ferritic stainless steel (BCC); when carbon is added to allow martensitic hardening (cf. Chapter 8), it can be used for knives and razor blades. Fe+18%Cr+8%Ni would make an austenitic stainless steel (FCC); this is used for flatware and automobile trim, for example.

Exercises

1.1 Aside from cost considerations, is a good way to protect iron against corrosion? Explain your answer. 1.2 How would this differ from zinc coating (galvanizing) the iron? 1.3 What is the risk of connecting a new copper water to an existing. iron pipe? How can this risk be reduced, while continuing to use the same two dissimilar pipes? 1.4 Copper and are often plated with nickel, as are the brass nipples used to attach spokes to wheel rims. (Further examples are plumbing fixtures such as faucets.) Explain the purpose of the nickel and describe how it functions. 1.5 When a piece of carbon steel is exposed to humid air, rust forms all over the surface. Therefore, localized anodes and cathodic regions must exist on the surface of the piece. Speculate on what might cause this. 1.6 The decorative steel straps on the steel plates bordering the South Street bridge near the University of Pennsylvania campus (Fig. 1.1 0) have bulged out as a result of crevice corrosion behind them. From the discussion of pit­ ting corrosion, explain in terms of oxygen concentrations why the rust formed under the straps. (This represents a direct conversion of chemical energy into mechanical work.)

11 CHA.P TER I CORROSION AND CORROSION PROTECTION

Fig. 1.10. Decorative steel straps on steel plates that border the Sou.th Street bridge lead­ ing to the Penn campus. The straps have buckled out from the rust that formed beneath them owing to crevice corrosion. 1.7 1 Zinc anodes weighing 1OOg are attached to the steel hull of a ship to pro­ tect it from corrosion. If an average corrosion current of 0.01A is measured through each anode, how long will they last? Suggest a reason why zinc anodes are normally used for this purpose, instead of, say, magnesium or cadmium; give a basis for your answer. (See Appendix 2 at back of book for useful data.) 1.8 Explain why using copper rivets in a steel plate would cause less of a cor­ rosion problem than steel rivets in a copper plate. 1.9 Write the anode reactions and use simple sketches to show the path of flow of electrons for corrosion under the following three conditions: (a) A zinc-coated carbon-steel spoke with a gap in the coating. (b) A nickel-coated carbon-steel spoke with a gap in the coating. (b) Two overlapping carbon-steel plates held together by a carbon-steel bolt and nut. (No coatings on any of them.) 1.10 How would you evaluate the feasibility of making stainless-type steel spokes using an Fe-Al alloy or an Fe-Ti alloy, in the event of a scarcity of chromium. 1.11 What is the total number of atoms contained within an FCC unit cell? What ·is the packing density (i.e., the fraction of the unit-cell volume occupied by atoms, using the hard-sphere model)? Note: The atoms touch along face diagonals. 1.12 A common carbon steel used for spokes contains 0.4wt% carbon. Convert this to atomic %carbon. (Use the Periodic Table for atomic weights.) 1.13 Aluminum costs more per ton than does steel, but aluminum has virtually replaced steel for beverage containers. Give as many factors as you can think of that could explain this replacement. 1 Contributed by Professor D. L. Callahan, Rice University.