Emerging Technology for Enhanced Corrosion Resistance
Posted on Thursday, January 7, 2021
By Kevin Biller
Powder coatings, by their nature, possess rather good corrosion resistance. This is due to robust polymers, careful attention to metal cleaning and preparation, and in general, thicker coating films than most industrial liquid coatings. Still, in some more extreme environments, primer/topcoat systems are needed to provide a combination of excellent UV durability and requisite corrosion resistance. In other cases, specifications call for maximum corrosion resistance due to severe service requirements and longevity expectations. For example, buried oil and gas pipelines are expected to last upwards of 30 or 40 years and longer. This installment of Technology Interchange will provide an update on the latest developments in enhanced corrosion resistance in powder coatings.
The performance of zinc-rich primers is well known and has a history that predates some of the earliest powder coatings technology. Liquid based zinc-rich primers have been around for over 50 years with proven corrosion performance. Powder formulators piggy backed on this technology to provide high performance coatings for extremely corrosive conditions such as coastal and marine environments. The most common powder coating approach to combat these highly corrosive environments is the use of a zinc-rich primer followed by a UV-durable topcoat.
However, powder coaters that use zinc-rich powder primers can attest to the difficulty experienced with applying and recycling these high specific gravity powders. Moreover, the coverage per square foot is relatively low. In addition, high zinc content in a powder can lead to increased wear of internal parts of the powder application system such as venturis, transfer hoses, and gun tips. Recent developments by powder producers have steered away from the use of zinc as a corrosion resistance formulation technique to a new generation of high performance non-zinc powder coatings.
Corrosion Resistance Categories
Defining corrosive environments is a critical component in specifying and selecting the appropriate powder to meet the rigors expected in the life of a coated product. ISO 12944-6 provides an excellent delineation of typical environments where corrosion occurs. Its categories span from C1, essentially a dry indoor environment, to C5-M, highly corrosive coastal and marine conditions.
Global suppliers of powder coatings use these classifications when recommending and warranting coating systems. In other cases specifiers develop their own requirements and describe them in a formal specification. These typically involve qualifying coatings per test protocols such as ASTM B-117 Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM D-2247 Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity, and cycle corrosion tests such as GM 14872 and ASTM D-5894.
Recent Powder Coating Developments
Axalta Alesta® ZeroZinc
Axalta has developed a family of powder coating products dubbed Alesta® ZeroZinc that provide excellent corrosion resistance without the inclusion of zinc in the formulation.
This product line has options for a variety of substrates including ZeroZinc Steel Prime for heavy ferrous metal parts; ZeroZinc Edge Prime, a powder designed with a beefier viscosity profile for parts with sharp edges; ZeroZinc Antigassing Prime designed for substrates that emit gas during cure such as metallized or galvanized steel; and ZeroZinc Antigassing Reactive for heavy gauge parts prone to gassing during cure.
Axalta touts the Alesta® ZeroZinc product family as primers for the architectural (metallic structure, urban furniture, and ironwork), transportation (bodies and equipment), industrial machinery, and agricultural equipment markets. These epoxy-based products are capable of delivering corrosion protection up to C5-I according to ISO 12944-6 standard, and because they have a much lower specific gravity than zinc rich primers, offer higher coverage per pound and reduced wear of powder application components (i.e., venturis and gun electrodes). Each of these primers provide excellent intercoat adhesion with Alesta topcoats.
PPG ENVIROCRON™ Extreme Protection Edge
PPG ENVIROCRON™ Extreme Protection Edge powder is claimed to deliver exceptional edge corrosion protection in one coat. These outdoor durable coatings have been specially formulated to cover sharp edges created during metal fabrication and provide coverage that outperforms standard one-coat and two-coat powder systems. Direct-to-metal ENVIROCRON™ Extreme Protection Edge powder coatings require no primer and are fully reclaimable, which can offer significant savings in material, labor, utilities, and time.
Applicators can also save by avoiding the need to finish edges with mechanical edge rounding or blasting equipment. PPG asserts that ENVIROCRON™ Extreme Protection Edge powder coatings have the potential to reduce warranty claims and improve resale value by extending the product lifecycle. The new coatings are available in a full range of standard and custom-matched colors, including mica and metallics and can be applied using manual and automatic powder coating systems.
AkzoNobel - Interpon® Redox Powder Systems
AkzoNobel offers a comprehensive line of powder coating systems designed specifically for corrosion protection in moderate to extreme environments, ranging from swimming pools to chemical plants and highly corrosive coastal and marine areas. The four main products are Interpon® Redox Active, providing excellent edge protection and a broad curing window; Interpon® Redox Plus, offering durability over a variety of substrates and metal pretreatments; Interpon® Redox PZ, specifically formulated for blasted steel surfaces; and Interpon® Redox Triplex, which consists of a three-layer coating system for the ultimate in durability and corrosion protection in harsh environments up to ISO 12944-6 Category C5.
The new Interpon® Redox range is suitable for everything from bus shelters, elevators, and security gates, to wind turbines, metal fences, and agricultural equipment. Initially introduced in Europe, Interpon® Redox was recently launched in Asia and North America.
Advances in Raw Materials Designed for Corrosion Resistance
A multitude of corrosion inhibitors exist in the liquid paint formulating world. The use of anti-corrosive pigments based on chromates, phosphates, titanates, and silicates is a well-known and evolving technology in liquid paint formulating. These compounds work by chemically passivating the substrate, whereas high concentrations of zinc work as a sacrificial anode consuming moisture and electrolytes before they attack the iron in steel. Chromates and phosphates have negative impacts on the environment and are being replaced by other less hazardous compounds such as silicates, zirconates, and titanates.
When formulated in powder coatings, these materials generally offer only modest improvements in corrosion resistance. Powder coatings generally provide excellent corrosion resistance through a combination of a tough polymeric network that establishes a robust barrier to ions and outstanding adhesion to the substrate. In addition, the judicious selection of extender pigments (function fillers) further enhances corrosion protection by creating a circuitous path for ions to travel through the coating to the substrate. Platy materials (consisting of plates or flaky layers) such a mica and kaolin clay as well as acicular (needle-shaped) minerals such as wollastonite are commonly used to enhance corrosion resistance of a powder coating formulation.
Innovative approaches to further the corrosion resistance of powder coatings have recently been proffered by raw material suppliers, including resin manufacturers and additive producers. Some are directed at primers, mainly epoxy-based powders, whereas others are focused on beefing up the corrosion resistance of UV-durable polymers such as industrial and superdurable polyesters.
Polymer scientists at DSM have developed a series of outdoor-durable polyester resins with enhanced corrosion resistance that are claimed in many cases to eliminate the need for epoxy powder primers. The Uralac® Corres family of resins offers outstanding corrosion resistance and barrier properties by working as a barrier for transport of water and oxygen through the coating. They also provide excellent adhesion to metal substrates, including untreated bare metal or substrates beset with lower quality pretreatment. These unique resins can be formulated into polyester-based primers and also as single-layer topcoats—where they offer superior adhesion to substrates along with excellent UV resistance.
The first product from this line, Uralac® P 7604 is specifically developed for topcoat and primer applications and provides exceptionally good corrosion protection. As part of a topcoat formulation, Uralac® Corres technology offers the possibility to significantly reduce and sometimes completely eliminate chemical pretreatment steps. This offers significant economics for the powder coater in terms of processing costs, chemical wastes streams, and application system capacity.
Formulated as a primer, Uralac® Corres resins can replace epoxies. DSM claims that the performance of an epoxy-free powder based on Uralac® Corres is equal to, or better than, many conventional epoxy-based systems, but with a much better environmental footprint. As an additional benefit, Uralac® Corres reduces the risk of delaminating between layers—a traditional drawback of epoxy-based primers. Whether used as a topcoat or as a primer application, Uralac® Corres is a polyester resin for corrosion protective coatings that increases operational efficiency in an eco-friendly way.
Allnex recently introduced a new range of CRYLCOAT® polyester powder coating resins for high corrosion protection on untreated and pretreated metal for a myriad of architectural and industrial applications. These five new Allnex products are specially designed to offer improved corrosion resistance when applied either direct-to-metal or on pretreated substrates. The systems are available for both HAA (hydroxy-alkyl amide) and TGIC (triglycidyl isocyanurate) curing agents for Qualicoat Class 1 (one year Florida) and Class 2 (three years Florida) weathering resistance. This range of polyester resins includes products designed specifically for ACE (agricultural, construction, and earthmoving equipment), architectural, and industrial equipment markets.
Allnex testing of powder coatings formulated with these polyesters resins exhibits significant improvement in corrosion resistance. Over shot-blasted steel all powders outperformed unmodified controls after 500 hours exposure of ASTM B-117 salt spray. In addition, powders evaluated over zinc-phosphated cold rolled steel withstood up to 2000 hours salt spray exposure. Allnex claims these improvements were made by establishing better barrier properties in conjunction with improved overall adhesion to the substrates tested.
Not only are the resin manufacturers offering innovation in enhanced corrosion resistance, the specialty additive producers are getting into the act as well. Micro Powders, a specialty wax and additives supplier to the powder industry, has developed GraphShield 730, a corrosion inhibitor based on a wax blend of graphene oxide. Graphene’s platy particle morphology improves corrosion resistance by establishing a tortuous path for moisture and ions to travel through a coating matrix. However, graphene, being a nano-sized particle, is difficult to disperse into a powder coating formula. Micro Powders has mitigated this problem by pre-dispersing graphene in a homogeneous synthetic wax blend. The synthetic wax melts during powder coating extrusion and uniformly distributes the pre-dispersed graphene oxide nanoplatelets throughout the powder formulation.
Test results show that dosages of 1-3% provide a dramatic improvement in corrosion resistance over both bare steel and iron phosphated cold rolled steel based on ASTM B-117 salt spray exposure.
The adage “rust never sleeps,” coined by the prolific rock musician Neil Young, may have little to do with protecting metal hardware from the ravages of a corrosive environment. However, in the powder coating world, material scientists and coatings formulators are the ones staying up at night. Their creativity and recent technical breakthroughs are putting corrosion to bed.
Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.