Coatings – Product Coatings
Overview
Considerable industry research on the corrosion protection of rock bolts has been undertaken and is still ongoing as new processes and coatings appear in the market. In addition to coatings, DSI Underground also offer a range of patented double corrosion products in cable and solid bolts, for use with cementitious grouts for long term protection from adverse environments. Some product coatings can change the coefficient of friction on the surface of the rock bolt and therefore affect load transfer at the interface of rock bolt and resin or cement grout interface. The following information is provided to illustrate options available to protect DSI Underground ground support elements and offer general awareness to the effects coatings and processes may have on function and performance of the ground support systems.Painting
Epoxy paints are generally considered the best for roof support applications particularly due to excellent hardness. Other paint options include polyurethanes, silicones, zinc rich and non-convertible coatings such as vinyl, acrylic latex, chlorinated rubber and grease. Resistance to acids, alkalis, water, solvents and heat can vary, comparative information is available on request from DSI Underground to aid choice along with hardness, gloss and colour variations. Conditions that may suit paintingWhere short term corrosion protection is required and prevention of mechanical damage to the painted surface can be assured. Situations that may not suit painting
- Where mechanical damage to the painted surface is likely to be encountered.
- Where reducing the coefficient of friction with a painted surface does not significantly impact on the performance of the roof support.
Hot Dip Galvanising
This process involves immersing the steel products in molten zinc resulting in a metallurgical bonded alloy coating that protects the steel from corrosion. The alloy coating consists of zinc-iron layers. Some of these layers are harder than the base steel, resulting in hot dip galvanising providing good resistance to abrasion. Conditions that may suit hot dip galvanising Hot dip galvanised coating acts as good corrosion protection over a range of 6 to 12.5pH. Situations that may not suit hot dip galvanising- In conditions with the pH range of below pH 6 (acid) or above pH 12.5 (alkaline) hot dip galvanised coatings are ineffective and rapid corrosion of the galvanised layers will occur.
- Enclosed tube products such as OMEGA-BOLT® cannot be hot dip as any entrapped water will boil instantly and could cause the tube to explode in the molten zinc bath.
- Steels susceptible to hydrogen embrittlement. Hydrogen embrittlement is a process by which various susceptible metals, most importantly high strength steel (over 800 MPa), become brittle and fracture following exposure to hydrogen. This can arise from hydrogen in the acid pickling solution penetrating the steel surface during the hot dip galvanising process.
- Chemical composition of steel; silicon and phosphorus composition can affect coating mass and appearance.
- Hot dip galvanising can sometimes clog up threads making it difficult to screw on fittings.
Painted Galvanised Coatings (Duplex Systems)
Painting galvanised steel gives added chemical & corrosion resistance in extreme service. It is essential that only compatible paint systems specified as suitable for galvanised steel are used, and that the pretreatment, primer, and coating sequence is followed exactly. Conditions that may suit duplex systems Where an increase in galvanised coating life is required. Situations that may not suit duplex systems- Where low cost is required.
- Where mechanical damage to the painted surface is likely to be encountered.
- Where reducing the coefficient of friction with a painted surface impacts on the performance of the roof support.
Stainless Steel
Steels are said to be stainless when they resist corrosion. This is achieved by dissolving sufficient chromium in the iron to produce a regenerating chromium oxide protective film on the surface. Selection of correct grades of stainless steel is critical and can need expert confirmation. Conditions that may suit stainless steel Where cost is not inhibitive and on site pH testing has shown stainless steel is suitable. Situations that may not suit stainless steel Different stainless steel grades perform better than others depending on conditions. There are multiple cases in industry where incompatible stainless steel grade selection has resulted in earlier than expected corrosion. Generally speaking the high cost of stainless steels mean that they are rarely used in large ground support projects.Thermal Diffusion Galvanising
This process provides a metallurgical reaction between the base metal and the zinc to occur. Zinc-iron alloy is formed on the steel’s surface at temperatures below the melting point of zinc. The steel items to be coated are in intimate contact with the specially activated zinc powder that is used in the process. The items are usually rotated in a heated kiln with all surfaces including internal threaded surfaces uniformly coated. The thermal diffusion coating thickness is closely controlled by process control systems. Conditions that may suit thermal diffusion galvanising- Where enclosed tubes need to be coated such as
OMEGA-BOLT®. - Where hydrogen embrittlement is a concern as this process does not include acid pickling.
- Where threads need to be coated.
- Where environmental considerations are prevalent as these plants produce less emissions than hot dip galvanising plants due to no acids or chromates in the process.
- In conditions with the pH range of below pH 6 (acid) or above pH 12.5 (alkaline) galvanised coatings are ineffective and rapid corrosion of the galvanised layers will occur.
- Where high volume applications already work successfully with hot dip galvanising.