With a vast selection of coatings and other protective products available in the market and new, higher-tech solutions on the way, mine operators can choose from many effective weapons to blunt the attack of a relentless chemical enemy
By Russell A. Carter, Managing Editor
Given the nature of the business, the twin forces of corrosion and abrasion are almost constant companions in mining operations. The typical extraction sequence of blasting, loading, hauling, crushing and grinding chunks of raw dirt and rock—often under harsh environmental conditions—basically constitutes an informal demonstration laboratory for the mechanisms of material destruction at thousands of mines around the world. And, since few locations have exactly the same geological, environmental, structural or operational characteristics, the struggle to combat damage to infrastructure and equipment from corrosion is generally a localized battle.
The site-specific nature of corrosion problems and solutions may be the reason why estimates of the annual cost of corrosion in the mining industry vary widely. The actual cost figure may not be critically important, but it’s high; a 2001 report from the U.S. Federal Highway Administration, for example, estimated that the overall annual cost of corrosion to critical sectors of U.S. infrastructure was more than $150 billion. More recently, a study by the National Association of Corrosion Engineers (NACE) pegs the total annual cost for organic and metallic protective coatings in the U.S. alone at $108.6 billion.
The NACE’s technical literature suggests that no material is resistant to all corrosive situations but materials selection is critical to preventing many types of failures. Factors that influence materials selection are corrosion resistance in the environment, availability of design and test data, mechanical properties, cost, availability, maintainability, compatibility with other system components, life expectancy, reliability and appearance.
Appropriate system design is also important for effective corrosion control, and includes the consideration of many factors such as materials selection, process and construction parameters, geometry for drainage, avoidance or electrical separation of dissimilar metals, avoiding or sealing of crevices, corrosion allowance, operating lifetime, and maintenance and inspection requirements.
The bottom line: Experts believe 50% of all corrosion costs are preventable, and approximately 85% of these fall within the area of coatings. Industry vendors such as Sherwin Williams Protective & Marine Coatings, Akzo Nobel’s International Paint, Dulux Protective Coatings and others supply a steady stream of new and improved anti-corrosion coating products each year, and many of these can be remarkably effective and long-lasting—if they are applied correctly, on a surface that has been properly prepared.
Test, Treat and Finish
Phoenix, Arizona, USA-based ChlorRid International points out that protective coatings designed for steel, for example, perform best when applied to a salt-free, clean surface. If the steel is coated before any one of a number of harmful soluble salts is removed from the surface, corrosion and premature failure of the coating are likely. Left unchecked, salt contamination corrodes deep into the substrate, making future decontamination even more challenging. For this reason, a number of industrial and military organizations have established tighter limits for acceptable soluble salt content on surfaces prepared for coating.
ChlorRid has developed two salt testing kits: its ChlorIon Meter is a hand-held digital testing device that electronically measures chloride with an internal ion-specific electrode; ChlorTest is a complete system that the company said will allow even inexperienced inspectors to obtain accurate results. Once tested, surfaces can be cleaned by applying ChlorRid’s salt removal product.
Products and methods to test and remove salt contaminants are available from a variety of sources.
However, not every coating installation can take place under ideal conditions, and for that reason Sherwin-Williams added a new product to its Macropoxy family of high performance coatings—Macropoxy 80, a high-build HAPs-free epoxy formulated for application over marginally prepared steel substrates and damp surfaces. The coating, according to the company, combats corrosion from both immersion and atmospheric exposures and can be applied at temperatures as low as 0°F (-18°C).
A modified phenalkamine epoxy, Macropoxy 80 is recommended for use on water and wastewater tanks as well as structural steel, pipe, tanks and other equipment in industrial applications. Because of its surface tolerance, Macropoxy 80 can be applied in adverse conditions, and steel substrates need only to be cleaned of loose paint or rust (per SSPC SP2-3 Hand and Power Tool Cleaning) before application. Its high solids formulation (80%) reduces the likelihood of solvent entrapment that can lead to premature coating failure.
Peel-off, or Permanent
Not all coatings are made to last forever. For example, Spraylat International, a U.K.-based company, offers the Protectapeel line of environmentally friendly coatings that are applied as a liquid by spray or roller and dry to form a skin-tight, plastic film that can be easily stripped away after use.
According to the company, Protectapeel is often ideal for protection of metal during shipping or storage. The product is claimed to prevent rust from forming on the protected surface; when the items are ready to be installed or delivered the coating can simply be stripped off by hand, avoiding some common problems encountered with other protective products such as the need for solvent or hot-water cleaning to remove sticky residue, waxes or oils before the protected item can be used or installed.
All Protectapeel coatings are water-based, making them safe for users and eliminating any need for special safety wear when working with the product.
Other coatings are designed to be practically—or completely—invisible. For bearings and metal parts that are exposed directly to humid or corrosive environments, NKE Austria GmbH offers a special galvanic coating that provides cost-effective protection against corrosion. The new and improved version of its SQ171E coating is thinner and provides longer-lasting protection against corrosion than the previous version, according to the company.
SQ171E coating can be used for standard or special bearings as well as for all metal parts exposed to wet or corrosive environments. According to NKE, even machined surfaces such as races can be coated, and the treatment provides protection against water, condensation and slightly alkaline or acidic cleaning agents. Compared to uncoated components, parts coated with SQ171E are claimed to have a significantly longer service life. As an additional option for even more effective protection the coating is also available with a silicate-based sealing layer. Due to the reduced thickness of the coating (from 2 to 4 µm) coated and uncoated parts are completely interchangeable. When compared to stainless steel, NKE says the SQ171E coating is more cost-effective, yet offers better anti-corrosion protection.
Meanwhile, according to results published recently from a joint university research project, a coating so thin it’s invisible to the human eye has been shown to make copper nearly 100 times more resistant to corrosion, offering tremendous potential for metal protection even in harsh environments.
In a paper published in the September 2012 issue of Carbon, researchers from Monash University in Australia and Rice University in the United States said their findings could mean paradigm changes in the development of anti-corrosion coatings by using extremely thin graphene films. Graphene, a substance with atoms arranged in a regular hexagonal pattern similar to graphite, but in a one-atom-thick sheet, reportedly has a variety of potential applications, including lightweight, thin, flexible yet durable display screens, electric circuits, and solar cells, as well as for enhancement of various medical, chemical and industrial processes. It also is attracting research attention for its possibilities as a means of increasing metal’s resistance to corrosion.
“We have obtained one of the best improvements that have been reported so far,” said study co-author Dr. Mainak Majumder. “At this point we are almost 100 times better than untreated copper.”
Dr. Parama Banerjee, who performed most of the experiments for the study, said graphene had excellent mechanical properties and great strength. The polymer coatings often used on metals can be scratched, compromising their protective ability, but the invisible layer of graphene—although it changes neither the feel nor the appearance of the metal—is much harder to damage. “I call it a magic material,” Banerjee said.
The researchers applied the graphene to copper using a technique known as chemical vapor deposition, and tested it in saline water. “In nations like Australia, where we are surrounded by ocean, it is particularly significant that such an atomically thin coating can provide protection in that environment,” Banerjee said. Initial experiments were confined to copper, but Banerjee said research was already under way on using the same technique with other metals.
Although the process is still in the laboratory-testing stage, Majumder said the group was not only looking at different metals, but also investigating ways of applying the coating at lower temperatures, which would simplify production and enhance market potential.
While breakthroughs such as these are intriguing and offer the prospect of significantly enhanced corrosion protection for a variety of materials and equipment types, the most cost-effective solution for mining applications isn’t always the most high-tech method or product. On the following pages, author Joseph P. Langemeier offers a persuasive argument in favor of hot dip galvanizing—a process that was conceived and patented almost 200 years ago.