The Australian Mining Review
CORROSION CONTROL FOR MINES
Corrosion of physical infrastructure vehicles, and machinery at mining operations and manufacturing facilities must be managed effectively to maintain safe and profitable operations.
ANY equipment failure can be expensive in terms of lost production and cost of repairs.
An estimate for the cost of lost production for a single dragline is $8000 per hour in Australia, making any breakdown very costly when it might take weeks for a replacement part to be available.
Machinery used to access, remove and haul away coal and other mineral resources at mine sites is often used during every shift and is under constant operational stress due to the sheer weight of the material or the rotation and vibration of components and engines.
Draglines and shovels are two key pieces of operating equipment with many shafts, pins, bores and lugs on them.
The corrosion of physical infrastructure at a mine site—mills, tanks, foundations, bunds—also has to be managed.
“There is a lot of concrete on a mine site that needs to be protected,” Extrin principal corrosion engineer Dr Peter Farinha said.
“This is often more important in terms of asset values when they need to be replaced.”
The operating environment of both vehicles and equipment impact their effective service life.
Variable climatic conditions can be damaging and the harsh physical environments of remote mine sites also take their toll.
These conditions include the dust, rocks and unmade roads in the arid, desert heat of Outback Australia through to cold alpine highlands and humid tropical coasts.
In some parts of Papua New Guinea and New Zealand equipment has to operate in sulphurous steam found in active tectonic areas.
The massive tyres of haulage trucks are extremely expensive to replace.
“A rock scratch can potentially expose the metal reinforcing which then can corrode,” IAS Group WA principal corrosion and coating engineer Graham Carlisle said.
“However, it is now possible to patch-repair damaged tyres with urethane-based elastomeric compounds.”
Ground water used in many mines for processing and equipment washdown is usually hyper-saline.
“Such water is extremely aggressive and damaging to infrastructure,” Mr Carlisle said.
“The high concentrations of chlorides rapidly seep into the concrete and begin to degrade the embedded reinforcing steel.
“Mine site ground water is often three times as salty as sea water, so mobile equipment on the ground cops a real hammering,” Dr Farinha said.
“Acidic gasses cause premature corrosion so there is not much left of your four-wheel drive after two years.”
According to Dr Farinha, areas of equipment and structures that are prone to rapid corrosion—but are often overlooked—are cable looms and connectors.
One way to protect the external surfaces of vehicles and machinery is to use spray applied surface protection, such as a polyurea or polyurethane.
Such coatings can extend the service life of a vehicle and associated equipment, in addition to reducing maintenance and repair costs.
Mr Carlisle stated that it is not just the outside of vehicles and structures that can be coated.
Mines feature large numbers of high pressure and volume pumps for moving water and processing slurry about the facility.
“Mine fluid flow systems suffer from an accelerated ‘erosion corrosion’ process, where particulates in the water or slurry scour the internal surfaces of pipes and pumps,” he said.
A recent repair project Mr Carlisle’s company was involved in was the relining of a 13-tonne split-case pump housing.
“We were fortunate that we could remove the whole housing to prepare and coat the inner surfaces,” Mr Carlisle said.
“In many situations it is too costly to interrupt a process running 24-hours a day so repair and remediation requires careful planning.”
In some processes, some members of the fixed structure are under water and other surfaces are constantly damp, so the coating material chosen must be able to be applied in wet conditions and withstand submersion.
Mine sites are extremely dusty, with dust and dirt settling onto all surfaces throughout the facility.
Structural steel I-beams and angle-iron are prone to accelerated ‘crevice corrosion’ where moisture soaks into the accumulated dust and begins corroding the metal that supports heavy machinery or tanks.
“The corrosion is often not noticed until the structure is cleaned, but a simple remedy might be to install ‘shedder plates’ that allow the dust to easily slide off the metalwork,” Mr Carlisle said.
Another way to minimise losses is to continually monitor plant and machinery using non-destructive testing (NDT) methods to ascertain component health, and monitor mechanisms for damage.
However, it is important that inspections are performed with minimal down time or costs, so need to be planned and coordinated to ensure that replacements for critical parts are available when the maintenance is carried out.
NDT is a group of techniques used to detect discontinuities in materials or components without causing damage or permanently altering the article being inspected.
Outage and emergency repair costs can be a significant percentage of the total operating cost of a mine.
Finding a crack and repairing it before failure, or monitoring it until it needs replacement and ensuring that parts are ordered to arrive in time for the maintenance work to be carried out, saves both time and money.
To support industry, the Australasian Corrosion Association ( ACA) works with academia and companies and asset owners to research all aspects of corrosion to provide an extensive knowledge base that supports best practice in corrosion management, thus ensuring all impacts of corrosion are responsibly managed, the environment is protected, public safety enhanced and economies improved.
“While coatings continue to be used, the industry is also looking at the judicious use of more corrosion resistant materials, such as duplex stainless steel and aluminium alloys, in the design of plant and equipment.”
Dr Farinha added that the training courses and technical seminars presented by the ACA have generated much greater awareness of corrosion management techniques.
“This awareness is leading to improved maintenance of mine site assets as site staff understand the importance of the effective monitoring of corrosion and planning for maintenance work to be carried out,” he said.
A range of premium spray-applied lining and coating products that provide maximum protection against corrosion, scratches and dents is readily available.
The coatings form a permanent air and water-tight bond that inhibits rust, corrosion and surface abrasion.
It is vitally important that coating materials do not crack, warp or peel—even under extreme temperatures.
The structure of most polymers used for surface coatings has to be resistant to abrasion and chemical attack.
The material’s strength comes from the bonding and cross-linking of the resin and hardener.
Companies providing surface coating application often work on a diverse range of projects.
Asset owners usually have to replace those parts of their machinery in regular contact with abrasive material—particularly earth, rocks and slurry at a mine or crops on a farm—every year.
Coating such parts can extend the service life by several years, resulting in far less downtime.
According to Dr Farinha, coatings and linings are the most common corrosion management techniques used on mines but there is increasing use of cathodic protection systems to protect embedded steel, immersed steel and reinforced concrete infrastructure.
“While coatings continue to be used, the industry is also looking at the judicious use of more corrosion resistant materials, such as duplex stainless steel and aluminium alloys, in the design of plant and equipment,” he said.
With state-of-the-art technologies and a comprehensive range of instrumentation available, mine operators have access to a wide range of non-destructive testing and corrosion management systems that are designed to extend the operational life of vehicles and equipment and thereby reduce maintenance time and costs.
Companies supporting the mining industry are committed to the development of new technologies, products, and services that offer the best solutions to meet the needs of its customers.
“Mine site ground water is often three times as salty as sea water, so mobile equipment on the ground cops a real hammering.”