ZERON® 100 Fasteners
Regularly used for pipework systems carrying corrosive fluids, ZERON® 100 corrosion resistant fasteners are used when corrosion of the flange bolts occurs. As a fastener ZERON® 100 is available in three different grades. It is available as fasteners in the solution annealed form, FG grade and FLT grade. The FG and FLT grades have a higher yield and tensile strength in comparison to the solution annealed form which is a more cost effective procedure for manufacturing headed fasteners. This is mainly down to the fact that solution annealed bolts can be headed by a forging process followed by solution annealing, whereas FG and FLT grades are produced and machined from bar.
Carbon or low alloy steel bolts can be used with ZERON® 100 flanges, however corrosion will occur because of the galvanic couple between the steel and ZERON® 100, especially if used in a corrosive environment. Regularly used in submerged marine applications, ZERON® 100 has excellent resistance to corrosion in seawater.
Coatings can be applied to protect carbon or low alloy steel bolts. However when coupled with ZERON® 100 in chloride solutions metallic coatings will corrode quickly. Therefore coatings must be thin to be effective on fasteners, which can be easily damaged. The galvanic couple with ZERON® 100 can cause serious corrosion to uncoated bolts, which is why ZERON® 100 resistant bolts should be used.
In chloride environments electrical isolation is sometimes attempted using isolation flanges and bolts with non-metallic sleeves. These can be easily damaged which can lead to the electrical isolation being lost before galvanic corrosion occurring. Connection of the piping to a common earth can also reduce the efficiency of an isolation flange, however ZERON® 100 corrosion resistant bolts can eliminate this problem.
During subsea use fasteners will be exposed to the cathodic protection which is applied to all of the structure. Major concern has been expressed regarding the potential for hydrogen embrittlement of duplex stainless steel fasteners. 25% of Cr duplex stainless steels fasteners under cathodic protection have failed. However high strength has been achieved by ageing, which has shown to greatly increase the susceptibility of duplex stainless steels to hydrogen embrittlement. Light cold work helps to achieve the high strength of ZERON® 100 FG and FLT grades. Research indicates that the threshold strength for the initiation of hydrogen embrittlement cracks is close to 0.2% proof stress and it is advised loading bolts are between 60% and 70% of the proof stress. It’s at these levels that hydrogen embrittlement cracking will not occur.
ZERON® 100 fasteners can also be used with high alloy, corrosion resistant alloys as well as ZERON® 100 pipes and flanges. They have been used in a wide variety of applications where corrosion resistance is vital. ZERON® 100 has been used to attach buoyancy modules on subsea risers, anchor rubber fenders to concrete docks in a Middle East harbor and attach cast sections of tunnel lining to the London Underground.
Commonly associated with stainless steels galling is less of a problem with more highly alloyed austentic and duplex alloys. ZERON® 100 bolts have been used in submarine pumps for a number of years and it is common practice to coat fasteners with ~6μm of copper. This makes the task of dismantling the pumps for servicing a lot easier. Successful use has been reported during critical applications. Molybdenum disulphide lubricant should be avoided during increased temperatures services.
ZERON® 100 fasteners provide a corrosion resistant bolting material formulated to overcome the galvanic corrosion problems commonly associated with less corrosion resistant fastener materials like carbon steel.