Pathways to Professionalism Proceedings of the RCI 20th International Convention & Trade Show Miami Beach, Florida March 31 – April 5, 2005 © Roof Consultants Institute 1500 Sunday Drive, Suite 204 • Raleigh, NC 27607 Phone: 919-859-0742 • Fax: 919-859-1328 • http://www.rci-online.org Fastener Corrosion in ACQ and Other “Next Generation” Treated Lumber By Heinz Wieland Presented by Gary P. Martini SFS Intec • Wyomissing, PA ABSTRACT The EPA has mandated changes in the composition of wood preservatives for pressure treated lumber. These changes eliminate the use of arsenic, but allow the introduction of other, more corrosive compounds. Steel fasteners are at risk. This presentation will discuss the corrosive nature of “next generation” pressure treatments and the various fastener solutions for this problem. SPEAKER GARY P. MARTINI is the vice president of sales and marketing for one of the world’s largest manufacturers of fastening systems for commercial roofs. SFS Intec employs 60 people in engineering and design and dedicates these resources to solving the problems of the industry such as this one. The issue of ACQ and similar pressure treatment systems has been a focus of the company for the past year. Prior to joining SFS Intec, Mr. Martini served for ten years as the manager of alloy products for a major steel producer, focused on the development of corrosion- and abrasion-resistant grades of steel for the construction and defense markets. Martini and Wieland – 107 Martini and Wieland – 109 PROBLEM DEFINITION Building materials, including roofing products, trim attachments, and gutters are often connected to pressure-treated lumber by metallic fasteners such as nails or screws. Like most metalbased components, these connectors and fasteners are susceptible to corrosion. Corrosion is a time- and environment- dependent process. Unfortunately, corrosion is also an extremely complex process. It is common for the causes of corrosion, and thus the methods of corrosion prevention, to be misunderstood and thereby misapplied. The very nature of the supply chain for components to the building construction industry often feeds this trail of misinformation. This leads to well-intentioned applicators installing components that are doomed to a usable life far short of written warranties or implied system life. Fastener manufacturers offer only comparative information based upon experiences in like materials. Lumber treaters transfer the onus for knowledge to the applicator. Trim or membrane manufacturers rely on the knowledge and experience of fastener manufacturers. As a result, in practice, proper science is never applied by any party to back up guarantees implied to building owners. The very reason that lumber is preservative pressure treated is to avoid premature failure due to insect infestation and rot deterioration. To prevent failure of a lumber or timber structure, both the wood material and the connection devices have to be stable over the intended period of service. Wood preservative treatment is an old process and there is significant practical experience with the behavior of metal fasteners in treated lumber. The reason for the renewed debate is an EPA- (Environmental Protection Agency) mandated change in lumber and timber preservative materials. The traditional preservative CCA has been replaced to a great extent due to health and environmental concerns. CCA is an abbreviation for copper chrome arsenate. Chromium and arsenic are harmful both to human health and environment. For purposes of this discussion, this is stipulated even though when properly applied and installed, the composite CCA has a rather low potential for harm. Key to this point is the proper allowance for time and/or process to thoroughly dry these materials. In defense of the new U.S. EPA regulations, we must acknowledge the fact that such time and treatment prior to application are the exception in the fast-paced manufacture-to-market cycle now in place in the U.S. Beginning with materials produced in January 2004, the EPA has banned CCA for residential use, whereas for some commercial use it still is permitted. The practical problem is that pressure treaters serve large markets and gain certain economies of scale by producing only a single more broadly accepted treatment type. Since CCA is not acceptable for all applications, it is simply more cost effective for producers to tool their manufacturing facilities for the exclusive production of the new, broadly permissible ACQ type material. There are several trade substitutes for CCA. The most common are: • ACQ – Alkaline Copper Quartenary (or as a shortcut, simply Quat) • CA or CBA (-A), CA-B – Copper (Boron) Azole • SBX – Sodium Borate (not recommended for all applications) The compositions of these new preservatives are roughly as follows: • ACQ solution: 49% copper oxide, 33% quaternary ammonium • Copper Azole (CBA-A): 49% copper oxide, 49% boric Acid, 2% tebuconazol There are various qualities of ACQ in the market; nevertheless, the copper content in both ACQ and CA is substantially higher than in CCA. Conclusions based on simplified theoretical corrosion analysis and tests indicate that fasteners corrode more rapidly in lumber treated with the new CCA substitutes. Fastener Corrosion in ACQ and Other “Next Generation” Treated Lumber Martini and Wieland – 110 Primarily as a result of the higher copper content, these new wood preservatives are more expensive than older CCA. This means that beside the technical problem of corrosion, there is a potential economic problem. Treated lumber is likely to become more expensive than it was prior to 2004, resulting in a longer return of investment period. This could have the effect of necessitating that the fasteners perform with intended values for a longer time than before, despite the increased corrosion risk. This concept should add to the immediacy and concern for this problem. For purposes of this discussion, we refer to these “next generation” pressure treatments simply as “ACQ,” as has become the industry terminology, even though CA is also a widely used material. UNDERSTANDING CORROSION Regardless of how corrosion is defined – galvanic, atmospheric, erosion, intergranular stress corrosion – there is one thing that is certain. Corrosion is a bad thing. It must be staved off or delayed using all available knowledge and methods. This is only possible when a scientific approach is taken to the corrosion issues at play in a given situation. Appropriate remedies must be applied to a problem. To do this, an understanding of the corrosive nature of the ACQ-treated lumber as well as the materials or coatings available for use in fastening requirements is necessary. There are several practical questions that must be understood in order to apply some certainty to a material or coating solution. 1. What level of corrosion tolerance does the application call for? Is a fastener effectively corroded if surface rust occurs, or the substrate or exposed surface is stained? 2. What is the application environment to which the coupling of lumber and fasteners will be exposed? Will it be closed, sheltered or exposed to clean air, salt spray, coastal, industrial, hazardous, or acidic conditions? 3. What type and concentration of preservative will be in use? The approach to a solution for this complex fastener corrosion problem is critical. Seeking the lowest cost solution while trying to cover each of the parameters at play can lead to minor misjudgments that in fact result in total system failure. Because of the clear need to combat all of the potential corrosion concerns in these materials, an appearance of “overkill” may be necessary. Material specifiers should keep this in mind when confronted with information from fastener producers claiming effectiveness against one particular aspect of ACQ-related corrosion. As clearly stated above, corrosion is a very complex problem; the above-cited parameters are still only part of the puzzle. When seeking answers to specific potentially corrosive application questions, it is common to receive a simple answer like: “Understanding the galvanic scale will tell you if you have a potential problem. Simply seek materials that are electrochemically similar, and you won’t have any problem.” But be very careful. This simple approach is very basic and while true under certain circumstances, it is impossible with limited information such as knowledge of materials specified for a particular project, to make this claim. The results, depending on other conditions, can be completely the opposite of what is anticipated. In fact, with such limited information, a specifier might as well simply select the lowest cost materials and hope for the best. So what is the galvanic scale, and what is it useful for? All metal materials have a specific electrochemical potential. This potential is generally thought of in the material’s most basic application, such as pure water from clean rain, absent acid rain or salt spray. But most people using these materials don’t truly understand how significantly these potentials can change under very real, everyday conditions. The electrochemical potential of the metals listed in the adjacent table is measured in a cell that consists of the metal strip submerged into a solution containing its dissolved ions and a platinum electrode submerged in an aqueous solution containing hydrogen ions. Voltage is measured between these two elements. This sounds very theoretical – and it is. Such a standard scale is limited to pure metals. It excludes metal alloys. Pure metals are very rarely used in construction applications. For more practical use, potential is measured in the same predefined aqueous solution for all metals, alloys included. Among these three solutions, not only are the potentials very different, but the sequence too. Look for example at steel versus zinc. In the pH 6.0 water series in the left column, zinc has a much lower potential than steel. In the pH 7.5 seawater series, the opposite is true. This is the reason that you will not find hot-dipped, galvanized components on oceangoing vessels. Therefore, if you are confronted with a suggested corMartini and Wieland – 111 rosion application solution using only the potential scales, ask your supplier in what aqueous solution the scale was established. In the case of ACQ, if the proposed corrosion resistance approach is measured in an ACQ solution with a limited predefined concentration, it may help to learn about the interaction between copper and a stainless steel part. But you will still not be able to determine the expected effect on a zinc-coated carbon steel fastener in an ACQ-treated piece of lumber. Readers may have also noticed in the table that in the standard potential scale, steel is missing. This is because steel is actually an alloy of iron and carbon. Alloys are not part of the standard potential scale. But nearly all metallic parts in contact with treated lumber are alloys. Even the more elaborate graph adjacent, showing the corrosion potential in flowing seawater at 10 to 27°C (50 to 80°F) in volts vs. standard hydrogen (upper scale), saturated Cu/CuSO4 (middle scale) and saturated Calomel [Hg/HgCl (lower scale)], is of little help, because most lumber is probably not intended to be used in flowing seawater. This elemental chart may, however, be of some value to corrosion specialists in that the very low potential of Ni-Cr-Mo steel may be of some interest. Austenitic stainless steels such as types 304 and 316 are Ni-Cr-Mo steels. In reality, one hardly has an anode – cathode situation when connecting treated lumber with fasteners as the situation in which the values above are measured. An anode – cathode situation means that a metal with a higher electrochemical potential and one with a lower electrochemical potential are electrically connected by an aqueous solution. In the above example, the seawater Practical electrochemical Series in water pH 6.0 Electrochemical series for theoretical elements ( standard potentials) Practical electrochemical series in sea water pH 7.5 Metal E° mV Metal E° mV Metal E° mV Silver 195 Silver 799 Silver 149 Copper 140 Copper 345 Nickel 46 Nickel 118 Lead -130 Copper 10 Aluminum -169 Tin -140 Lead -259 Tin -175 Nickel -230 Zinc (Zn 8 .5) -284 Lead -283 Cadmium 400 Steel -335 Steel -350 Iron 440 Cadmium -519 Cadmium -574 Zinc (Zn 8.5) -760 Aluminum -667 Zinc (Zn 8.5) -823 Aluminum – 1660 Tin -809 Potentials compared to a st andard hydrogen electro de E°mV = Electrochemical potential in millivolts Martini and Wieland – 112 was the aqueous solution. In a typical anode – cathode situation, the anode (low electrochemical potential – e.g., zinc) dissolves and the cathode (high electrochemical potential – e.g., copper) grows in mass as it is plated with the dissolved metallic ions in the aqueous solution. This principle is precisely the method used to intentionally electrically deposit metals such as chromium or zinc on steel components (plating). In practice, this phenomenon is also dependent upon the surface ratio of the two electrodes, the conductibility of the aqueous solution, and the external contact of the electrodes. Often, there is substantial real-world variation from the theoretical expected corrosion of metals. This is due to the dramatic variability of environmental conditions as described above. As a result, we must rely on simulation tests to approximate the true, reasonable expected behavior of fasteners in ACQ-treated lumber. Yet there is a serious problem with corrosion tests as well. Realistic tests may take years for results to appear, and accelerated tests lack standardization. One might suggest that if tests take years for corrosion to appear, then this is in effect a good sign for the material being tested. This is very far from the truth. In fact, the only truly perfect test for simulated corrosion testing is an exact replica of materials being tested under the exact application environment, for the full expected life of a connection. This means that a 20-year roof would have to be simulated for twenty full years. Reasonable shorter cycle comparisons may be made, but the nature of corrosion of protection coated materials (such as zincplated or epoxy e-coated) is that though the period prior to surface breech may be extended, the effect of the corrosive compound on the eventually unprotected steel may be dramatic and extremely rapid. Accelerated tests such as salt spray will produce interesting results if a project is planned for a coastal area or northern urban area where road salt is commonly applied. But how would this compare with test results of a non-cyclic, high humidity test in a Kesternich cabinet? The latter test would make sense in an industrialized zone. Unfortunately, there is a temptation to use one or the other test method for a special brand of fastener or surface coat that gives the most advantageous results compared with those of a competitor. Still, such non-cyclic humidity test results, as shown below, may give some indications about the probable behavior of zinccoated fasteners in wood treated with the different preservatives. But one should be aware that zinc coatings vary in type and technique of manufacture. The chart of tested wood material reflects the results of actual tests performed at SFS Intec labs in Heerbrugg, Switzerland. It shows the corrosive rates of zinc-plated fasteners in lumber with the listed treatment types, relative to the base process – untreated pine lumber. Nevertheless, results of a comprehensive series of tests of noncyclic, high humidity Kesternich tests do allow for a better understanding of what happens to fasteners in ACQ-treated timber. First, the test series proves that, in general, corrosion of metallic parts in ACQ-treated lumber is stronger than in CCA-treated timber. This is a key point. Regardless of what else we are able to determine, and absent any long-term real-life experience, we should be immediately alarmed by this fact. Most specifiers would be hesitant to make assurances about life expectation for a product known to be of lesser corrosion resistance, with nothing more definitive to establish an opinion. Second, all types of zinc-plated fasteners begin corroding by producing white rust – a popular name for zinc oxide. Therefore, zinc does not simply dissolve, but oxidizes. At about the same rate, stainless steel fasteners – 300 and 400 series – become copper plated while carbon steel-based fasteners begin to corrode. This means that the copper in ACQ, which is in the form of copper oxide, gets reduced to pure copper and plates the stainless steel fastener, which acts as a cathode. This has no adverse effect on the corrosion resistance of the stainless materiMartini and Wieland – 113 al. At the same time, however, the zinc coating on the carbon steel fastener oxidizes and no longer provides the necessary protective layer for the steel. The theoretical effect is that the carbon steel begins to corrode, producing red rust or iron oxides. This is precisely what is observed in lab tests. Needless to say, no copper plating occurs on the stainless steel in untreated lumber and in only minimal copper plating occurs on the stainless in CCAtreated lumber. The tests, therefore, help us to better understand what happens in treated lumber. The process is not the same as with screws in metal structures. Here, the main corrosion effect is not of galvanic type but of oxygen type, also known as general corrosion. The galvanic mechanism occurs to a smaller extent, depending upon the materials being used. From this we can draw several conclusions. First, standard zinc coatings are of little use. Thick zinc coats, such as heavy hot dip galvanizing, may help for a limited time. Epoxy barrier coatings such as commonly used roofing screw e-coat (electromagnetic coatings) will have the same minimal effect. Other improved zinc coats may also show a slowed corrosion rate, but zinc oxide corrosion seems to be unavoidable and with it the fast degradation of zinc. Zinc layers exposed to the humidity of the atmosphere will corrode as well, but the corrosion product is a zinc carbonate (due to the carbon dioxide in the air) – a strong, non-abrasive protective layer that slows corrosion to a large extent. Zinc oxide, or white rust, has no such effect. Again, tests support this theoretical expectation. Fasteners that were unscrewed for periodic inspection exhibited much more rapid corrosion than fasteners left in place. The zinc oxide was abraded and lost. Although it offered only a minimal protection function, its degradation resulted in accelerated fastener corrosion. While this oxide removal process accelerates corrosion, it does not promote it, and therefore we can accept the observations of these fasteners as a prediction of the inevitable increased corrosion of the undisturbed test fasteners. The conclusion is that austenitic (300 series) stainless steel fasteners should be used, as they are virtually unaffected by the wood preservative. If they are plated with a thin zinc layer, it will only serve to provide lubricity for installation rather than to improve corrosion resistance. After installation, the zinc has no function and may corrode away, leaving the austenitic stainless steel to fend off the corrosive effects of ACQ on its own, which it will do quite effectively. Martensitic (400 series) Stainless Steel There remains a question. Does martensitic stainless steel (such as type 410) not perform as well against ACQ? In terms of true surface corrosion, neither martensitic series 400 nor austenitic series 300 (304 and 316) fasteners showed visible degradation in industry ACQ tests. But the 400 series (martensitic) fasteners produced some stain on the surface. This is to be expected and will naturally be a problem where fastener heads are visible on the structure surface. Staining may produce clearly visible spots that dissipate into the lumber, leaving the appearance that something is wrong with the fastener. Beyond cosmetic concerns, 400 Series martensitic stainless steel fasteners are thermally hardened and are more brittle than 300 Series austenitic stainless steel fasteners. 400 series stainless steels such as type 410 use elevated carbon levels to achieve hardenability. This is at the sacrifice of corrosion resistance properties. Further, there is significant evidence that 400 series stainless steels are susceptible to an altogether different type of corrosion – the phenomenon of stress corrosion. While this is an entire subject of its own, put simply, depending on the environment in which they are used, these materials may develop intergranular corrosion while under load, ultimately leading to fastener failure. Therefore, for purposes of this paper, we only consider austenitic varieties (300 series) as a viable consideration as a solution to the ACQ question. RECOMMENDATIONS We have noted that ACQ- or CA-treated wood may be more expensive than the traditional CCA-treated lumber. Logically, the payback period is longer and the service life of fasteners may be expected to be longer. For zinccoated carbon steel fasteners, the opposite applies. They corrode faster in ACQ- or CA-treated wood. The already existing disparity between the expected service Standard Zn-plated carbon steel fastener after 154 hours in ACQ lumber chamber. Martini and Wieland – 114 lives of lumber and carbon steel fasteners increases – not in favor of the fasteners. To gain a service life equilibrium between ACQ- or CA-treated lumber and the fasteners used to join it, it is strongly and exclusively recommended that austenitic (300 series) stainless steel fasteners be used. This will also serve to eliminate the ugly staining on the lumber or other material surface. Even such fasteners with a carbon steel tip for improved installation will be effective, provided that adequate fastener length is specified to ensure that the integrity of the fastener remains long after the carbon steel tip is corroded away. Under an industry test, fasteners were left for 154 days in a Kesternich cabinet with pure water humidity at a temperature around 104°F degrees. The two adjacent images show carbon steel and austenitic stainless steel fasteners after the simulation. Whereas the carbon steel fasteners were heavily corroded to a condition where structural safety would be compromised, the austenitic stainless steel series 300 fasteners showed very little corrosion. The head of the stainless fastener exhibited some white rust caused by the zinc coat that is applied for installation lubricity, and the carbon steel tip exhibited some stain and red rust. This tip is designed and expected to fail over time and the head and shank are protected, as they are comprised of stainless steel under the thin zinc coating. Corrosion, therefore, is limited to zones where it produces no harm. CONCLUSION ACQ/CA pressure treatments are proven to be more corrosive than traditional CCA wood treatments. Galvanic as well as oxygen- related corrosion occurs in these materials. Because of the variable environments in application as well as the condition of wood material at delivery (moisture content variability), corrosion prevention cannot be accomplished through improved workmanship or care. Further, due to high variability in materials and conditions, existing coatings such as zinc and epoxy e-coats are likely to be of little long-term effect in reliably preventing this corrosion. Certainty can only be gained through the use of austenitic stainless steels for fastening these materials. Austenitic stainless steel fasteners are more costly than coated carbon or martensitic stainless steel fasteners. This cost difference, however, pales in comparison to the potential catastrophic costs of roof, panel, trim, or other structural failure due to misapplied carbon steel fasteners in ACQ- or CA-pressure treated lumber materials. Specifiers must be cautious and mindful of available industry information that is clearly designed to support proprietary products and treatments. Extensive studies and data must be insisted upon. As discussed herein, there are no simple answers to complex corrosion issues such as ACQ. Zinc plate carbon steel 304 stainless steel with carbon tip Fasteners after 154 days in non-cyclic Kesternich high humidity chamber, with periodic removal for inspection:
