Methods and Developments in Zinc-Based Anticorrosion Coatings

COATINGS & LININGS

M. Gore and S. SMith, Greenkote PLC, Corrosion, particularly on iron and sion. The HDG process can accommodate a Brook Park, Ohio steel components and structures, very wide range of part sizes—from con- represents a very significant cost to struction beams and angle irons to nuts society. Corrosion-resistant coatings and bolts. Complicated shapes such as fas- have become vital for helping to miti- tener threads, however, can suffer from nonuniform coating, and require rework on gate the impact of corrosion in many precision features. Another issue is that industries, including water systems, small components are often cleaned by oil and gas, construction, military, pickling in preparation for HDG, and this automotive, and aircraft. Today, the can introduce the risk of hydrogen embrit- issues and concerns extend well be- tlement (HE) to the part. Recent innova- yond corrosion. As users seek im- tions, such as specialized alloying addi- proved corrosion protection, now tives, can be utilized to enhance HDG’s they are also looking for metal coat- corrosion resistance, and techniques such ings that can provide enhanced prop- as spin galvanizing can improve the thick- erties such as resistance to wear and ness uniformity. Overall, HDG remains a galling, better thickness uniformity, capital-intensive and heavily regulated pro- stronger surface adhesion, greater af- cess because of its use of acids during pickling, zinc vaporizing from the molten pool, fordability, and resistance to hydro- and leaching of lead. gen embrittlement of high-strength steel, all while being eco-friendly. Zinc Electroplating Another widely used coating process is zinc electroplating, or electrogalvanizing. Zinc-based coatings, one of the oldest Electroplating involves placing a steel part anticorrosion methods, is still among the and a zinc anode in an alkaline or acidic most commonly used. Hot-dip galvanizing zinc bath through which a current is passed (HDG) is one of the most widely used corro- (Figure 2). Zinc from the anode then depos- sion-resistant coatings. It involves dipping its onto the surface of the steel part (cath- a fabricated steel part into a pool of molten ode) and builds a continuous layer. The zinc (Figure 1). HDG was developed in the thickness can be precisely controlled by the 1700s and continues to be used in many magnitude and duration of applied current, industries. It is cost effective and provides but the deposition is generally thinner than a thick coating to all exposed surfaces. The HDG because it is a slower process. Uni- Zmolten zinc metallurgically bonds to the form coatings can be applied to compli- surface of the part and provides good adhe- cated shapes, but features such as recesses

34 JANUARY 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 1 and sharp corners can enhance or limit coating thickness as the coating grows, leading to nonuniform or limited coverage. This is due to nonuniform current density at such features—often referred to as the Faraday effect. Because the bonding mech- anism is chemical in nature, the adhesion can be limited under some conditions. However, the bright, shiny appearance of zinc electroplating is sometimes consid- ered more appealing than HDG. Zinc electroplating is often less expensive than HDG; however, it generally does not offer the same level of corrosion protection because of the thinner coating. Also, the electrolysis reactions during cleaning and plating often generate hydrogen in high- strength steels, which can be a risk or addi- tional cost to mitigate. Various passivation chemistries may be FIGURE 1 HDG of fabricated steel parts. added after electrogalvanizing to improve corrosion resistance, appearance, or other properties of the coated part. Zinc alloys, 70% compared to HDG or electrogalvaniz- including those with Ni, Co, and Fe, can ing). Coating uniformity can be an issue as also be used to enhance anticorrosion well. High spots tend to wear in the properties. Proprietary combinations of mechanical tumbling action, and areas that alloy chemistry, combined with passivation cannot receive the mechanical peening treatments, can provide good corrosion action, such as holes, have limited opportu- protection and visual appearance. How- nity to receive a zinc layer. Mechanical zinc ever, as with HDG, the electrogalvanizing is still used on high-strength steels, as it industry is heavily regulated because of its can be applied with no exposure to hydro- use of acid/alkaline chemicals, the large gen and no risk of embrittlement. amounts of water consumed or treated, In spray metallization, zinc powder or and the heavy metals used in alloying and wire is melted with some type of heat passivation treatments. The overall envi- source, such as an electrical arc, gas FIGURE 2 Zinc electroplating diagram. ronmental concerns associated with plat- plasma, or oxyacetylene flame, and sprayed ing operations are potentially very severe. directly onto a metal component at high velocity (Figure 3). The partially or fully that bridges the gap between metallic and Zinc Powder Methods molten zinc particles impact the part sur- organic coatings. Zinc dust can be mixed Zinc powder can be applied directly to face and solidify into a coating that is with an organic or inorganic binder and, metal parts through mechanical plating or denser than mechanically peened zinc and similar to paint, be brushed onto the sur- spray metallization. In the mechanical can approach 90% density. However, it still face of a part or structure to yield a zinc- plating process, parts are tumbled in a is not as dense as HDG or plating, and its rich surface layer when dried or cured. The rotating drum with proprietary activation adhesion is primarily mechanical in nature. resulting low-density coating provides chemicals and a tumbling medium such as One advantage of spray metallizing is that some degree of barrier and sacrificial pro- glass beads. In many instances, the compo- it can be applied in the field to coat welded tection to the substrate. However, like nents are given a prior flash plating layer as or assembled structures or repair other gal- paint, the zinc coating’s corrosion protec- part of the overall cleaning and preparation vanized coatings. tion and adhesion will depend on how well process. The zinc powder, which is the substrate’s surface is cleaned and pre- mechanically peened onto the surface of Zinc Flake and Paint Coatings pared. A technically advanced form of this the part, forms a protective layer of zinc; Processes such as zinc flake or zinc hybrid coating, often referred to as zinc but at a much lower density (typically 65 to paint represent a hybrid type of coating or metal flake coating, is comprised of

NACE INTERNATIONAL: VOL. 56, NO. 1 MATERIALS PERFORMANCE JANUARY 2017 35 COATINGS & LININGS

called diffusion coating or Sherardizing, invented in the early 1900s by Sherard Cowper-Coles. In the original diffusion coating process, finished metal components were placed into an enclosed cylin- drical drum containing a filler medium, typically sand, to which zinc dust was added. The drum was sealed, mechanically rotated, and placed into a kiln to heat the mixture to approach the melting point of zinc (419 °C). At this temperature, the zinc dust vaporized and diffused into the steel parts, building a layer of zinc-iron interme- tallics across all exposed surfaces. The Sherardizing process yielded satis- factory corrosion protection for certain applications, but the coating systems were complex and expensive and the coatings’ thickness, uniformity, and density were dif- ficult to control. Continuous charging of the zinc raw material tended to result in inclusions or regions in the coating with high iron content as well as sand entrap- FIGURE 3 Thermal spray coating. ment, which led to inconsistent corrosion protection properties. Modern furnace designs, often with proprietary formulations with zinc powder radiant heating as well as simplification of materials in combination with organic and the drum loading and charging process, inorganic binders and sometimes metal have improved efficiency and ease of use powders such as aluminum. These zinc for the zinc TD coating process while dra- flake coatings are applied using dip-spin or matically reducing capital costs. Compa- precision spray techniques to create a uni- nies such as Greenkote, PLC are supplying form coating, and they often employ spe- this newer generation of zinc TD coating cialized base layer and topcoat combina- worldwide. tions to address specific applications and environments. The resulting coatings can Modern Zinc Thermal provide high levels of corrosion protection Diffusion Coatings with the added advantage that binders can Consistent, predictable zinc TD coat- be colored to provide a range of decorative ings in the range of 10 to 50 µm currently finishes. As with all barrier-type coatings, can be applied with a combination of very zinc flake coatings can be vulnerable to good uniformity and density, even on parts mechanical damage and breakdown from with complex geometries. With proper extended exposure to ultraviolet light and drum/retort design, parts of many shapes weather. The solvents and other carriers and sizes, from a few millimeters to several used to apply these coatings also create meters in length, can be coated with zinc environmental issues during the curing TD. Batch sizes can range from 100 to over process, although there is an increasing 5,000 kg. Finely tuned control of the tem- push toward water-based carrier systems. FIGURE 4 Comparison of zinc coatings after perature profile, coating material charge, 500 h of salt spray testing. retort rotation, and process times can effi- Zinc Thermal ciently process many different ferrous Diffusion Coating materials in wrought, cast, or powder A zinc thermal diffusion (TD) coating metal formats. Further innovations in represents a newer, more technically equipment design have eliminated the advanced version of an older process need for sand or other filler media, so the

36 JANUARY 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 1 Methods and Developments in Zinc-Based Anticorrosion Coatings

batch size is maximized and coating inclusions are minimized. Specialized zinc-alloy chemistries also have been developed to accelerate the reaction, promote full density, and enhance corrosion resistance of the coating by generating a more robust alloyed surface oxide layer. Modern zinc TD processing can provide excellent corrosion resistance for steel, in part because zinc-iron intermetallic structures intrinsically have better corrosion resistance than pure zinc or metallic zinc layers (Figure 4). However, the advantages of a TD coating go well beyond basic corrosion resistance: since the coating is inte- grally diffused into the surface of the part and provides a true metallurgical bond, coated parts can withstand deformation and still maintain corrosion protection. In addition, the Zn-Fe intermetallic structure is inherently harder than traditional metallic zinc coatings, in the range of HRC 40, FIGURE 5 Surface of a zinc TD coating showing its characteristic microroughness. which provides enhanced wear and abra- sion resistance and makes the coated part significantly more damage-tolerant in the field. This increased surface hardness also process uses no solvents, acids, or complex erage of complex shapes, and elimination reduces the potential for galling. gases and requires very little process water of HE—all while remaining eco-friendly. except for post-cleaning, and even that Since the surface coating fully reacts MARK J. GORE is president and chief with the steel surface metallurgically, the water can be cleaned and reused. The pro- executive officer of Greenkote Plc, 6435 coating is thermally stable up to the range cess does not involve any dangerous chemi- Eastland Rd., Brook Park, OH 44142, cals and contains no heavy metals such as e-mail: [email protected]. He has of 600 °C. Because the coating is created by 30 years of experience in the areas of continued evaporation of zinc in close Cr, Ni, Cd, or Co, which are experiencing coatings and surface modification technol- proximity, even complex surface geome- increased regulatory scrutiny. The only ogies in the steel, battery, synthetic process waste is a minimal amount of zinc diamond, semiconductor, oil and gas, and tries can be uniformly coated, including anticorrosion industries. He has a B.S. recesses and even internal diameters. The oxide (ZnO) dust, which can be collected degree in metallurgical engineering from TD process also allows assembled compo- and recycled or sold as a raw material to the University of Cincinnati and a Ph.D. in other industries. metallurgy from the Massachusetts nents, such as chains or hinges, to be effec- Institute of Technology. A member of tively coated while fully assembled. In addi- NACE International, Gore is a member of tion, the zinc TD coating surface features a Conclusions the ASTM Metallic-Coated Iron and Steel Products A.05 Technical Committee. unique, natural microroughness (Green- Because of its strength, ductility, and kote†, Figure 5) that enhances the adhesion toughness, as well as recyclability, steel is SEAN SMITH is the engineering manager of subsequent surface layers. Paints or likely to remain the material of choice for a at Greenkote Plc, e-mail: sean.smith@ other organic and inorganic layers can be greenkote.com. He has worked in the great many applications for years to come. foundry industry for 10 years as a metallur- applied to the TD surface with little or no While corrosion is still a significant vulner- gist and operations manager. He joined special surface preparation. For the same ability for steel, modern zinc-based coat- Greenkote in early 2015. He has a B.S. reason, zinc TD surfaces are well suited for degree in materials science and engineer- ings continue to mitigate the problem and ing from The Ohio State University. He is a rubber overmolding and other subsequent extend the life of this very useful metal. member of NACE International. bonding applications. Developments in zinc-based protective coatings are ongoing, and advances for the Read MP Online Environmental Issues zinc TD coating in particular have posi- NACE International members have free In contrast to most of the other zinc tioned it to play an even stronger role in the access to MP Online, with back issues to coating processes, zinc TD is inherently future. This new generation of coating is 2005. clean and environmentally benign. The bringing superior corrosion protection Access the digital issues of MP at along with increased wear and galling resis- materialsperformance.com. †Trade name. tance, excellent adhesion, conformal cov-

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