Corrosion Principles and Galvanic Reaction

Home , Structural steel

This article has been published in two parts. Part I appeared in the July 2013 edition of Interface and discussed basic corrosion principles and galvanic reaction. Part II explains why some metals are more corrosion-resistant and their reactions in cementitious materials.

CORROSIONͳRESISTANT METALS Carolina, a friend (believing that stainless Just as all stainless steels do not have Stainless Steel steel was low-maintenance and rust-proof) the same corrosion-resistant properties, Even though stainless steel is positioned purchased one of the new stainless-steel they do not have the same fabrication prop at the “noble” end of the Galvanic Series Ta revolvers to keep in the cabin of a fishing erties such as welding, bending, hardening, bles, it does not occur naturally. The occa boat. There was considerable shock when, etc. Therefore, when considering fabrication sional references to it as being a noble metal over time, some of the shiny silver-looking and application of stainless steel, design are incorrect as determined by the previ metal had small pits and turned a cinna professionals should consult publications by ously discussed defi nition of a noble metal mon color. The lesson learned is that not all the International Molybdenum Association (see July 2013 Interface). There is a mis stainless steel is rust-proof. (IMOA) or Specialty Steel Industry of North conception that all stainless steel is alike The American Iron and Steel Institute America. and is corrosion-resistant. Stainless steel (AISI) lists over 50 types of stainless steel. is an excellent metal and should not get a These types are usually identifi ed by 200, Aluminum bad reputation because of poor application. 300, or 400 series numbers, with Types 301, Aluminum is not actually a corro Just as the term “wood” is used to embrace 304, 316, and 410 being those most frequent sion-resistant metal. Even though alumi a wide range of species, with each species ly associated with design and construction. num’s position on the Galvanic Series Table having different properties, there are differ • Austenitic: Types 200 and 300 indicates a reactive metal, more accurately, ent alloys of stainless steel and each alloy Series (304, 316) are nonmagnetic under most conditions, aluminum’s corro has its own properties. When the species of and have reasonably good corrosion sion rate is just extremely slow or is in a a particular wood is not known, that wood resistance. Type 304 (18% chromi static state. Aluminum is very active and is sometimes referred to as “tree wood.” um, 8% nickel) is the general-pur tends to oxidize quickly to form a white, Similarly, stainless steel is often specified pose alloy in this group and is often chalky passivating fi lm as shown in Photo as “stainless steel” with no mention of alloy. referred to as 18-8 stainless steel. 6. Actually, very few metals are more reac Just as some roofs perform better than • Martensitic: 400 Series, such as tive than aluminum, and those metals are others under specifi c conditions, the same Type 410, is the general-purpose not normally used in building construction. can be said for stainless steel. This is alloy of this group. It is very magnet Beryllium, potassium, sodium, and magne because some stainless-steel alloys are more ic and is corrosion-resistant in mild sium are examples. This rapid oxidation is corrosion-resistant than others. Therefore, atmospheres. a benefi t instead of a detriment, because the stainless-steel type, as determined by its • Ferritic: 400 Series, such as 430, the aluminum oxide fi lm forms a strong alloy, must be matched to its environment. I is magnetic and has good corrosion bond to the aluminum surface and seals recall that while living in Charleston, South resistance. the aluminum from oxygen. For this reason,

S EPTEMBER 2013 I NTERFACE • 29 it offers excellent resistance to corrosion and provides years of main tenance-free service in natural atmospheres. This is the opposite of common steel corrosion, where the oxidized metal (rust) fl akes off and exposes more metal to corrosion. According to the Aluminum Association, the galvanic process is a very common cause of aluminum corrosion. As long as the aluminum oxide remains intact, no further corrosion will occur. Because of its inherent resistance to other forms of corrosion, when aluminum comes into contact with less reactive metals, aluminum acts as a sacrificial anode and becomes susceptible to corrosion. At an ambient temperature of 80ºF, the normal surface fi lm for mation ranges from approximately 2 to 50 nanometers thick. If that protective layer is scratched or abraded, a protective fi lm re-forms immediately in most environments and ensures continued protection. Consequently, aluminum and its alloys can be used in a wide range of building applications that include fl ashings, copings, gutters, down spouts, roof panels, window frames, stairs, ladders, fences, railings, pipe, and many more applications other than construction. Although aluminum has a huge advantage compared to other met als regarding corrosion, it is not always completely immune to corrosive reactions. Its protective oxide layer can become unstable when exposed to extreme pH levels. A highly acidic or alkaline environment can break down the protective layer and make the aluminum more susceptible to corrosion. According to the U.S. Army Corps of Engineers, aluminum’s protective oxide fi lm is generally stable in the pH range of 4.5 to 8.5. Aluminum corrosion is not normally noticed in freshwater lakes, pools, etc., but it can become more obvious in or near the ocean. It may seem logical to conclude that the saltwater, because of its alkalinity, is corrosive to the aluminum. In reality, saltwater does not corrode alu minum because of its neutral pH. However, saltwater can become the electrolyte for galvanic corrosion. Just as various stainless steel alloys have different strengths and Photo 6 – Aluminum oxide on a coastal ladder. (Photo by Cris corrosion-resistant properties, so do aluminum alloys. When corrosion Crissinger.) resistance and strength are design factors, marine-grade aluminum

Figure 3 – Common detail. (By Homer Nestlen of McMillan Pazdan Figure 4 – Recommended detail. (By Homer Nestlen of McMillan Smith.) Pazdan Smith.)

30 • INTERFACE S EPTEMBER 2013 Photo 7 – Mortar expelled by expanded and corroded reinforcement. (Photo by Cris Crissinger.)

Photo 8 – Exposed masonry reinforcement. (Photo by Cris Crissinger.)

alloy such as 6061 or 6063 should be mation of patina considered, according to the Aluminum is desirable. Association. However, 7075 alloy has a Since copper signifi cantly higher strength than 6061 or is highly noble, 6063 but has inferior corrosion resistance. it is frequently used for most Atmospheric Corrosion of Copper atmospheric Since copper is a noble metal and is applications generally stable in most atmospheric con such as sheet ditions, it is often used for gutters, down metal work for roof and wall fl ashing, gut corrode and fall off. This cycle continues, spouts, roof panels, and trim for historic ters, downspouts, trim—and, sometimes— causing the metal to eventually disintegrate structures and is highly recommended for roof panels. However, copper should be completely. The volume increase created by through-wall fl ashing and other applica avoided in locations with elevated levels of the corrosion can push mortar out masonry tions where it will come into contact with sulfur or ammonia. If copper must be used joints and will eventually force the horizon moisture. in these environments, it can be treated tal reinforcement from the joints. Similarly, Proper detailing of copper through-wall with nickel-plating or tin-plating, which act corroding steel rebar can cause concrete to fl ashing can prevent galvanic action. Figure as oxide inhibitors. spall. Photo 7 shows mortar being pushed 3 is a common method of showing the leg from a masonry joint by corroding rein of a shelf angle turned up and the flashing STEEL CORROSION AND forcement, Photo 8 shows exposed reinforce carried over the anchor bolt. If the flashing CEMENTITIOUS CONSTRUCTION ment, and Photo 9 shows corroded masonry is metallic, such as copper, a potential gal Water from various sources and oxy anchors. In addition to steel corrosion, vanic cell is created between the steel bolt gen from the air (anode) and the copper (cathode). Figure 4 take many paths shows the leg turned down, preventing con to the interior of tact with the fl ashing. Since the steel angle the concrete or (anode) is signifi cantly larger than the cop concrete masonry per (cathode), the larger anode-to-cathode units to consume ratio reduces the corrosion potential of the underprotected steel. steel. The iron As copper corrodes, it develops a strong oxide formed by the self-adhering oxide layer similar to the reac corrosion usually tion of aluminum. Like aluminum, as the bonds to the steel, corrosion process advances, the layer thick loosely causing the ens and forms the familiar green patina original volume of often associated with copper roofs. Sulfur the steel to increase dioxide that forms in the atmosphere from many times. The burning fossil fuels can hasten the transi loosely bonded tion from oxide layer to patina. In applica corrosion falls off, tions such as roofi ng, where the green pati exposing more na has aesthetic value, the increase in for- metal that will also Photo 9 – Corroded masonry ties. (Photo by Cris Crissinger.)

S EPTEMBER 2013 I NTERFACE • 31 aluminum, it forms a white-powder pro tective coat (an oxide), sometimes referred to as white rust, when exposed to the ele ments. For instance, oxygen from the atmo sphere causes zinc to quickly change to zinc hydroxide; and carbon dioxide—also in the atmosphere—changes the zinc hydroxide to zinc carbonate, which stops the reaction and prevents further corrosion.

Galvanizing Galvanizing is a simple example of cathodic protection consisting of a protec tive coating of zinc and ferrous metal such as steel that are bonded together with no electrolyte. As long as the zinc coating is not breached, there is no reaction. When galvanizing is exposed to corrosion, it forms zinc’s characteristic white rust as shown in Photo 11, where the exposed steel edge is Photo 10 – Spalled concrete caused by expanded corroded rebar. (Photo by Bailey and Son beginning to corrode and the galvanizing is Engineering.) sacrifi cing itself. If the zinc fi lm is broken to expose the ferrous metal, the zinc will begin effl orescence is a very visible sign of saline warms the walls, the moisture also warms to sacrifi ce itself to prevent corrosion of activity in cementitious construction. Photo and begins to move as a vapor and increas the steel metal. The zinc coating is usually 10 shows spalled concrete and corroded es the corrosion potential. applied by one of the following two methods: rebar in a fertilizer plant. Excluding using • Hot-Dipped Process: Dipping the protective coatings such as galvanizing or ZINC, GALVANIZING, GALVALUME®, base (ferrous) metal in a vat of epoxy, corrosion of reinforcement in cemen AND ANODIZED ALUMINUM molten zinc, providing a thick, dull, titious construction can be controlled by: Zinc gray coat of zinc, usually with the 1. Not using ocean sand in mixes Zinc is a fairly reactive metal and is characteristic spangle. This is the 2. Using integral water-repellent in used in sheet-metal work, but is probably most common method of galvanizing mortar best known for its use in galvanizing. Like and provides the best protection 3. Not using admixtures that con tain chlorides 4. Using concrete with a low water-to-cement ratio 5. Tooling masonry joints to cre ate a smooth mortar matrix 6. Using dense concrete with smooth fi nish and thick cover over the rebar 7. Not pressure-washing mason ry, which can destroy the water-resistant mortar matrix 8. Using concrete and mortar materials and mixes with min imum proportions of alkali and sulfates

Since an electrolyte is a necessary component of corrosion and water can be an electrolyte, corrosion tends to occur where rainwater or conden sation cannot run off or evaporate quickly. Porous masonry can act like a giant sponge and absorb moisture from the atmosphere. Since heat tends to invigorate corrosion, when the sun Photo 11 – Corrosion (white rust) on galvanized steel edge. (Photo by Cris Crissinger.)

32 • INTERFACE S EPTEMBER 2013 because its coating is much thicker. If the applied area turns black, there is no Depending on the alloy of the ferrous passivator because of the chemical reaction metal being galvanized, the process between the copper sulfate and bare zinc. If may weaken the base metal some there is no color change at the applied area, what. Also, being a thicker coating, there is a passivator because the passiva hot-dipped galvanizing can fi ll the tor prevented a reaction. (Copper sulfate is threads of bolts—especially those of usually available from drug stores that still smaller diameters—thereby reduc compound prescriptions.) ing holding power of the connection. Additionally, hot-dipped galvanizing can • Electroplating: Producing a thin, be touched up with cold galvanizing coat shiny application of zinc coating, ings that have 90-95% zinc in the dry film sometimes resembling satin stain or zinc-rich epoxy primers that have a zinc less steel. Being a thin coat, the plat content of 80-85% zinc by weight. The cold ing tends to deplete itself faster than galvanizing coatings tend to work best hot-dipped zinc coating. Therefore, because they produce a dry fi lm that has it is not usually recommended for a higher zinc content, which means better exterior or wet applications unless sacrifi cial properties. painted or used in an arid climate. However, because of its relatively Galvalume® lower cost, it is frequently used in Galvalume® is a trade name that iden lieu of hot-dipped galvanizing. This tifi es cold-rolled steel sheet that has been type of plating is used for metal coated with an aluminum-zinc alloy. The studs and is common for nails used alloy consists of approximately 55% alu in nail guns. minum, 43.4% zinc, and 1.6% silicon by weight. However, aluminum makes up Corrosion resistance is directly propor approximately 80% of the alloy by volume. tional to coating thickness. The two most The coating is applied by dipping the cold- popular coating thicknesses for galvanized rolled steel sheet into the molten alloy, steel are designated G-60 and G-90. The G producing sheet steel having the protective designates hot-dipped galvanizing, and the properties of aluminum and zinc and the number designates the total amount of zinc barrier protection and longevity of alumi contained on each side of the sheet. G-60 num. The zinc also provides better corrosion contains 0.60 ounces of zinc per sq. ft., and resistance at the cut or sheared edges. G-90 contains 0.90 ounces of zinc. G-180 is The two most popular coating thick often recommended for substrates in con nesses for Galvalume® are designated as tact with treated lumber and has 1.80 oz. of AZ50 and AZ55; however, AZ60 is some zinc per sq. ft. times used. The AZ stands for aluminum Fabricators usually coat the surfaces zinc, and the number represents the total of zinc and its relatives, galvanizing and combined thickness of coating on both Galvalume®, with an oil to prevent the sides of the sheet. An AZ50 has a combined white crust from forming. These oils can coating thickness of 0.50 oz./sq. in. on both be a slip hazard on a galvanized roof deck sides, which is equivalent to approximately and must be removed from any surface to 1.6 mils. Table 4 correlates the thicknesses. be painted. In lieu of oil, fabricators may Both AZ-55 and AZ-50 coatings contain treat galvanizing with a passivator, which 55% aluminum, 43.4% zinc alloy, and 1.6% also must be removed prior to applying a silicon. The silicon minimizes the growth of protective coating. The oil is easily detected brittle intermetallic layers that form when by feel, but a passivator is not usually easily the product is being coated and allows the detected. A passivator can be detected by alloy to be applied by the hot-dipped pro applying copper sulfate to the galvanizing. cess. Galvalume’s® appearance is sim Oz./Sq. In. Total Mil Mils Each ilar to hot-dipped galvanizing but Both Sides Both Sides Side tends to be smoother and shinier, AZ50 0.50 1.60 0.8 with a smaller and tighter grain; and its slight spangle is not so prominent AZ55 0.55 1.76 0.88 as that of hot-dipped galvanizing. AZ60 0.60 1.92 .96 These properties tend to produce a smoother fi nish when they are coat Table 4 ed. Performance of hot-dipped galva-

S EPTEMBER 2013 I NTERFACE • 33 Photo 12 – Passivation (brown rust) on old riﬂe barrel. (Photo by Cris Crissinger.)

nizing and Galvalume® tend to be similar during the ﬁ rst ten years of service, with perhaps a slight edge to galvanizing. However, Galvalume® begins to out perform hot-dipped galva nizing after approximately ten years and continues to do so for 15 to 20 years or more. Galvalume® should not be used on, in, or around cementitious materials such as concrete plaster, concrete masonry units, or mortar, because the high alkalinity can react with the aluminum and cause accelerated corrosion. Hot- dipped galvanizing per forms better when exposed to cementitious conditions. However, hot- dipped galvanizing is not recommended for and does not perform well as a through- wall ﬂ ashing, because building movement can cause contact surfaces to abrade the galvanizing and expose the ferrous sub strate. Both coated and uncoated Galvalume® can deteriorate quickly when exposed to animal excrement, so they should not be used in or around areas that house livestock. However, coated and uncoated Galvalume® can be successfully used in marine and most industrial environments.

Anodized Aluminum Anodizing is a common process that increases aluminum’s corrosion and abra sion resistance and provides a chemically bonded color to the aluminum, but it is not the same as galvanizing, which is con sidered to be a coating. Since aluminum forms a natural layer of protective oxide that prevents or slows the rate of corrosion, anodizing artiﬁ cially thickens that natural oxide layer—often many times thicker than what would form naturally. This increased thickness provides additional protection. Unlike Galvalume® and galvanizing, anod izing is available in a small range of colors—all having similar performance.

34 • INTERFACE S EPTEMBER 2013 Photo 13 – Corrosion and rust bloom during construction. (Photo by Cris Crissinger.)

Photo 14 – Structural steel corrosion from protective coating failure. (Photo by Bailey and Son Engineering.)

The American Architectural Metals Association publication AAMA 607.1 classiﬁ es two types of clear anodized aluminum, based on coating thickness, as Class I and Class II. Class I has a coating thickness of 0.018 mm or thicker, and Class II has a coating thickness of 0.010 mm or thicker. AAMA 608.1 governs color-anodized alumi num.

CARBONATION OF CONCRETE AND STEEL CORROSION Carbonation is a deterio ration of the concrete and can cause surface porosity that can allow atmospheric conditions to reach the steel rebar and start corrosion and is directly pro portional to the porosity and moisture content of the con crete. Although it is not a form of corrosion, it can play a defi nite role in the the possibility for steel corrosion increases usually contained combinations of nitric rebar corrosion. When steel reinforcement is if the pH falls below approximately 10. A pH acid, copper sulfate, wine, and distilled encased in concrete or masonry, the steel is in the 8-9 range suggests that carbonation water (one formula even contained urine). protected by the concrete’s protective cover is taking place from the concrete surface The corrosive solution was carefully applied and the alkalinity of concrete or masonry. toward the interior. to the highly polished and squeaky-clean The alkaline in the concrete and mortar Once again, early Americans demon barrel and allowed to rust in an undisturbed causes a passivation fi lm to surround the strated their knowledge of metallurgy when location for a specifi c time, depending on steel, and that fi lm protects the steel from colonial gunsmiths applied the passivating desired results. The crust that formed by the environment the same way it does for process by using a controlled rusting proce the rusting was carefully removed to reveal aluminum, zinc, and copper. dure to produce a very desirable “butternut” a smooth, brown fi nish that protected the It takes a relatively high pH to protect brown fi nish on iron gun barrels as shown metal from further corrosion under normal the steel. Typically, the pH of concrete in Photo 12. Many early-American gun conditions. Bluing is a similar process. ranges from approximately 12 to 14, and smiths had secret browning formulas that When concrete is fresh, steel reinforce

S EPTEMBER 2013 I NTERFACE • 35 ment is protected from corrosion by the high substrate movement and thermal CONCLUSION alkalinity of the surrounding cement paste. stress Even though new alloys and protec The protective layer of concrete is stable and tive coatings are being developed, corro adherent in its normal range of alkalinity. Photos 13 and 14 show the results of sion principles have not changed since the However, the alkali in concrete eventually structural steel components that received a days sorcerers were practicing alchemy. reacts with acidic components in the atmo marginal shop primer and were not properly Corrosion cannot be completely eliminated, sphere, particularly carbon dioxide in the protected during on-site staging. Photo 14 but common sense and diligence can keep concrete. This reduces the alkalinity of the shows what happens when the corrosive it in check. If not controlled, a mild case of concrete by converting the calcium hydrox process in a textile plant breaks down the corrosion can become a pandemic. However, ide to calcium carbonate, which reduces the protective coatings and attacks the steel corrosion can be controlled by thoughtful pH value of the concrete below 10 where the structure. consideration of details, materials selection, concrete loses its protective ability. When In the unlikely event that extreme pH and ambient conditions. concrete or any other cementitious material levels or known corrosive chemicals are in contact with the embedded steel reinforc present and cannot be avoided, there are REFERENCES AND ADDITIONAL ing is carbonated, the steel surface loses its several simple solutions to avoid possible READING passivity protection. Now it is possible for damage, such as ammonization and cathod Joseph Bosich, Corrosion Prevention for corrosion to begin or resume when moisture ic protection. Practicing Engineers, Barnes and and oxygen gain access to the steel surface. Corrosion is a thirst monger, and swim Noble, 1970. The rate of carbonation is mainly influ ming pools are a Mecca for corrosion. It Cris Crissinger, “Considerations for enced by the permeability and the calci thrives on moisture—whether from an open Coastal Coatings, Part 1,” Interface, um content of the concrete, as well as by ing in a façade, from the atmosphere, or a RCI, Inc., March 2007. the ambient atmospheric conditions—the vapor from a structure’s normal breathing. Cris Crissinger, “Considerations for amount of carbon dioxide, relative humidi When selecting materials associated with Coastal Coatings, Part 2,” Interface, ty, and temperature. Also, concrete can car indoor swimming, wading, and therapeu RCI, Inc., June 2007. bonate more rapidly in a hot climate than in tic pools, careful consideration should be Cris Crissinger, “Considerations for a moderate climate. given to the ambient conditions around the Coastal Coatings, Part 3,” Interface, Edward Gerns’ article “Corrosion: The pool and chemical storage areas. Excluding RCI, Inc., July 2007. Use of Metal Within Masonry Wall Systems splash and spills, materials and compo Edward Gerns, “Corrosion: The Use of and Associated Life-Cycle Issues,” pub nents can be exposed to ambient air that is Metal Within Masonry Wall Systems lished in the March 2010 issue of Interface, warm, humid, and usually contains traces and Associated Life-Cycle Issues,” is an excellent resource of how ferrous met of the chemicals (often chlorine) used in the Interface, RCI, Inc., March 2010. als behave in masonry construction. pool water. These are ideal conditions for Charles G. Munger, Corrosion Prevention both galvanic and atmospheric corrosion. by Protective Coatings, National Protective Coatings Fountains are not normally heated, but Association of Corrosion Engineers Protective coatings provide a common they often contain antifungal chemicals; (NACE), 1984. method of corrosion control by separating and outdoor fountains usually contain anti- L.S. Van Delinder, Corrosion Basics: An corrosive materials from potentially corro freezing chemicals in addition to antifungal Introduction, NACE, 1984. sive conditions. All protective coatings will chemicals, all of which can be very corro Voluntary Guide Specification and provide some protection, but some do it sive. It is prudent to fi nd out the chemical Inspection Methods for Clear Anodic signifi cantly better than others. Anodized additives in the water and plan accordingly. Finishes for Architectural Aluminum, aluminum, galvanizing, and factory-applied AAMA A607.1, American Association Kynar®-type fi nishes can be included in the of Architectural Metals. protective coatings category. To be effective, a protective coating should have the follow ing properties: Joseph (“Cris”) Crissinger, CCS, CCCA, ASQ • Be matched to the environment and Joseph (“Cris”) Crissinger is semiretired and is a construction substrate specifi cations and materials consultant in Spartanburg, SC. • Be compatible with cathodic protec His responsibilities include construction specifi cations, mate tion if used rials analysis, fi eld investigations, and facility assessments. • Be abrasion-resistant to resist dam Crissinger is a Certifi ed Construction Specifi er and a Certified age from routine handling Construction Contracts Administrator. He is a member • Electrically isolate the substrate of the Construction Specifi cations Institute, the Building metal from corrosive conditions Performance Committee of ASTM, and the Design and • Resist deterioration due to the envi Construction Division of the American Society for Quality. He ronment and service temperature is a two-time recipient of RCI’s Richard Horowitz Award. He • Have suffi cient adhesion to resist also serves in the community on the Construction Board of Appeals for Spartanburg, under-fi lm migration of an electro SC, and is a U.S. submarine veteran. Crissinger may be reached at specalyze@reagan. lyte com. • Be fl exible to resist cracking from

36 • INTERFACE S EPTEMBER 2013