Uninsulated or bare piping resting directly on structural steel have resulted in repeated corrosion problems posing high risks for pipe leaks. The geometry of bare piping resting on structural steel provides a crevice that allows water to be trapped.
Since the paint system is designed to withstand atmospheric exposure and not immersion service, it softens over a period of time under the influence of trapped moisture. Eventually the pipe protective coating fails due to pipe movement on structural steel surface exposing the bare steel surface to water (which is often with high chlorides in a saline environment such as encountered in the middle east region or on offshore platforms) and starts to corrode. In some instances the failure of protective coating may have occured during installation of piping. In any case, the crevice holds moisture and consequently results in accelerated corrosion rate. It's worthwhile noting that even if the pipe and support are made from same material such as carbon steel, the metallurgical difference can provide a small potential difference to drive the formation of a corrosion cell. Crevice corrosion thus leads to a potentially hazardous situation for the piping system.
Additionally, the configuration of bare pipe resting on structural steel makes it difficult to carry out visual inspection of any damages to the protective coating and to carry out any maintenance repairs.
How can we prevent crevice corrosion under pipe supports
Some operators have tried to use neoprene or other rubber pads to isolate the pipe from structural steel. However, this solution is found to be counterproductive as the crevice formed due to deformed rubber has the ability to retian even more water and for a longer duration making the situation worse than before.
The solution lies in not allowing the root cause to occur i.e. the crevice to be formed. Operators worldwide have resorted to various measures to overcome this problem some of which are listed and further described below:
- Use of welded pipe shoes for pipe sizes 2" and above.
- Use of welded metallic re-pads or wear pads of material equivalent to parent pipe.
- Use of adhesive bonded non-metallic re-pads or wear pads.
Use of welded shoes for pipe sizes 2" and above
The welded shoe support is a viable solution. Though this option adds cost to the project in terms of additional material, fabrication and inspection, it offers the advantage of a robust design compared to other options. The shoe raises the elevation of the pipe and permits convenient access for inspection of the bottom of pipe as well as access to carry out repair of any damaged protective coating. Many operators in the middle east region have successfully implemented this solution on greeenfield projects. Some operators have even implemented an extensive program to identify and replace the type of supports on existing brownfield facilities.
Use of welded re-pads for pipe sizes 2" and above
The weld re-pad or wear pad is another viable solution to overcome the problem of crevice corrosion. In this approach, any crevice corrosion that is likely to occur is on the wear pad rather than on the pipe thus protecting the main pipe from further deterioration. Whenever a wear pad is used, it is important to ensure that the fillet weld is continuous to eliminate possibility of water ingress between the saddle and pipe. It is also vital to ensure that the weephole in the saddle is plugged after completion of the weld to eliminate ingress of fluid in the gap between the saddle and pipe. Another variation of this type of support is the use of corrosion bars fillet welded to the bottom of pipe.
Use of adhesive bonded non-metallic re-pads or wear pads
Another solution to counter crevice corrosion is the use of non-metallic wear pads such as plastic or FRP pipe cradles. The usual approach is to use preformed plastic cradles which are adhesively bonded to the pipe. A continuous seal is maintained around the cradle to rule out the possibility of any crevice being formed. The adhesive is required to be bonded and cured as per manufacturers recommendations. The limitation of this approach is the maximum temperature that the non-metallic wear pad can withstand. Viny Resin wear pads manufactured by Piping Technology & Products Inc. can withstand maximum temperature of 400°F. It is suggested to use welded shoes for higher operating temperatures to overcome crevice corrosion issues.
Other Solutions
There are several other products available in the market that have been successfully used to reduce the issues related to crevice corrosion at pipe supports. One such product is the I-Rod support which eliminates the potential of crevice corrosion by using half-rounded elements below the pipes.