Zinc-Based Coatings for Corrosion Protection

Coating systems are an engineering solution applied to improve surface properties. Depending on desired characteristics, substrate, and technical limitations, many different coating methods exist. On metallic parts, corrosion is one of the most critical engineering problems. For this reason, surface coatings protect the metal. Corrosion formation results from the metal's interactions and reactions with its environment. Because metals are conductive, electrochemical corrosion occurs in open atmosphere. To counter corrosion formation, repair, painting, and the use of corrosion-reducing chemicals become necessary, and these create significant maintenance and/or protection costs. Additionally, corrosion can indirectly cause damage to various metallic engineering structures, reduce system function and performance, and even lead to environmental problems. Based on the explanations above, increasing the corrosion resistance of metals through coating has gained considerable importance and interest in manufacturing. Depending on zinc's electrochemical potential relative to iron, Zn provides cathodic protection capability. Zinc-based coatings can be obtained through different methods. Each coating method has its own characteristic properties. In thermal, chemical, and mechanical coating processes, certain parameters affect coating quality and performance. Among these, temperature, duration, and the chemical composition of the solution or powder material should be considered. For this reason, care must be taken in selecting the coating method, and depending on desired properties and performance, the method, coating material selection, economic criteria, and process limits should be taken into account [1].

Uncoated and coated fasteners [2]

Each has its own specific characteristics and performance in the coating process, and there are many different zinc coatings used for corrosion protection. Zinc coatings include hot-dip galvanizing, electroplating, mechanical plating, and zinc enrichment on the surface of steel surfaces, as well as zinc-rich paint applications and zinc spraying (metallizing, thermal spray) methods. Of these, hot-dip galvanizing is the most widely used coating method. On the following page, a brief explanation of each zinc coating type is provided. The figure on the following page shows the typical thickness of each coating. In most cases, coating thickness is directly proportional to corrosion life.

Zinc coating methods and thicknesses [3]

 

a. Hot-Dip Galvanizing Process

In this process, parts whose surfaces have been pre-cleaned are galvanized by dipping them into a molten zinc bath (BS EN ISO 1461). Every surface of the part is completely coated, and depending on the mass of the galvanized steel, a layer of zinc and zinc-iron alloy is formed within a certain thickness range. This is the most important advantage of the galvanizing process—a standard coating thickness can be applied automatically, independent of operator involvement. The molten zinc in the galvanizing bath covers corners and edges on complex-shaped parts, seals joints and rivets, and can penetrate into recesses to provide complete protection against potential corrosion spots with other coating systems. Galvanized coating is somewhat thicker at corners and narrow edges, providing much more protection in these critical areas compared to other coatings. Complex shapes and open containers can be galvanized from both the inside and outside. From small fasteners to structures and products of varying sizes up to hundreds of meters in height can be protected against corrosion using this method [3].

Hot-dip galvanizing [6]

b. Continuous Galvanizing Lines

In these specially developed galvanizing processes, steel sheet, tube, and wire products can be galvanized on a continuous line (EN 10346, EN 10244-2, EN 10240). These processes are widely used in the industry and typically allow precise control of coating thickness and zinc coating; a wide range of products can be coated to meet the varying requirements of subsequent production, forming and processing operations, and end use. Products coated using this method should not be confused with bath-type hot-dip galvanized products. Products coated in continuous-line galvanizing always produce thinner coatings than hot-dip galvanizing for the same steel thickness and therefore provide less corrosion protection when exposed to the same environment. In continuous galvanizing lines, coated products can be processed, shaped through bending or roll forming, such as by clamps and bands, without damaging the coating. Welds, cut ends, and drilled or punched holes may require repair to provide corrosion protection depending on the application and environment [3].

Continuous galvanizing lines [7]

c. Thermal Spray Coatings (Metallizing)

The thermal spray or metallizing process (EN ISO 2063) is based on melting/semi-melting zinc or other metal alloys in wire or powder form within a heat source (flame, plasma, electric arc, etc.) and spraying. With zinc spraying, thicknesses of 250 microns and above can be achieved; in fact, up to 1,500 gr Zn per m² on the surface can be easily accumulated in an automated system or manual application in an open field or at an industrial scale in a factory. The surface must be grit-blasted before coating. There are limits in coating the interior surface of a part. Additionally, coating can be damaged or application can be limited at sharp edges, narrow corners, holes, and poor surface preparation. The zinc coating obtained provides both barrier and cathodic protection to the base steel in the same manner as a hot-dip galvanized coating. In most cases, thermal spraying is more expensive than hot-dip galvanizing for an equivalent surface, but these processes are complementary and can be used effectively on large structures [3].

Zinc metallizing

d. Electrolytic Zinc Coatings

The electrolytic zinc coating process (EN ISO 2081) is an economical, versatile, and effective method for small-sized steel parts. For fasteners, it is the most effective protective coating method in narrow cross-sections and threads. However, compared to hot-dip galvanized coatings, there are differences in corrosion properties. Generally, coating thickness and amount are limited; use in outdoor external conditions without an additional surface coating is not recommended [3].

Electrolytic zinc coatings [8]

Standards • AASHTO – AASHTO M298-97 • ASTM – ASTM B695-00 • ISO – ISO 12683 • Chrysler Corporation – PS-1536 • Ford – ESF-M1P37-A • General Motors – GM4344M & GM4345M • John Deere – JDM F22 • United States of America – MIL-C-81562B • RoHS Compliance

f. Zinc-Rich Paint Applications

Zinc-rich paint coatings consist of metallic zinc powder in organic or inorganic carriers/binders. Surface cleaning before coating is performed with abrasive grit blasting or surface grinding equipment. Coating can be applied by brush or spray. Zinc-rich coatings are barrier coatings that provide cathodic protection on small steel surfaces provided the steel surface is properly prepared. This application complies with relevant standards (for example, AS/NZS 3750.9 and AS/NZS 3750.15). Such paint coatings can also serve as a useful repair coating for damaged or worn galvanized coatings. These paints can be applied to components and structures in the field and of any size. In some applications, they can be applied in thicker layers or topcoats to provide extra protection. The disadvantages of these applications include curing times, friction damage during transport, scratching, and similar damage formation; at the same time, the cost for equivalent corrosion protection applications at appropriate thicknesses is somewhat high [3].

Zinc-based paints

g. Zinc Enrichment on the Surface via Sherardizing Process and Thermal Diffusion Method

Sherardizing (EN 13811:2003) involves heating steel products in a drum containing metallic zinc dust, usually to approximately 500°C. At temperatures above 300°C, zinc is vaporized and diffuses into the steel surface, forming Zn-Fe-containing intermetallic phases from the surface inward. Similarly, the thermal diffusion process (ASTM 1059) [10] is generally a more efficient process at lower temperatures (400°C) with less filler. This is not a new process; it was patented in 1890. Today it is still used in Europe. In both processes, for small-sized parts, they are quite suitable depending on drum sizes. These processes can also be safely used for very high-strength steels above 1000 MPa because they also prevent hydrogen embrittlement. Coating thickness varies between 20-120 μm, but is typically applied in the 20-50 μm range. Coating thickness typically depends on time in the rotating drum and is not dependent on steel thickness. These coatings provide salt spray test life of over 2000 hours [10-12].

Parts with thermal diffusion coating applied

Process comparison and standards [11]

Thermal diffusion zinc-based coatings are a process that can be used in improving and modifying the surface properties of metals and have application areas in many industries including automotive. Because it contains no harmful chemicals, it is an environmentally friendly technology and complies with automotive standards in the European market. This process in the automotive industry (VW, Audi, GM) meets both corrosion protection and low friction coefficient requirements. This method provides very high corrosion performance.

References [1] Metals Handbook,Vol 13.Corrosion.Edition 9, 1997. [2] http://fastenereurasia.com/ [3] https://www.gaa.com.au [4] Toshiaki Ohtsuka, Atsushi Nishikata, Masatoshi Sakairi, Koji Fushimi, Electrochemistry for Corrosion Fundamentals, Springer, 2018. [5] Handbook of Cathodic Corrosion Protection Theory and Practice of Electrochemical Protection Processes Book, 3rd Edition, 1997. [6] https://galco.ie/galvanizing-process/ [7] https://bigriversteel.com/products/galvanized/ [8] http://www.georgiaplating.com/zinc-electroplating.htm [9]http://www.ciprocessing.com/metal-finishing/mechanical-plating/ mechanical-vs-electroplating.html [10] http://greenkote.com/ [11] https://www.galvanizing.org.uk/metal-finishes/ [12] www.anochrome.com