There are a variety of factors at play. Here are a few in not particular order of importance but all of which must be considered.
1. Environmental Conditions: Are there restrictions in terms of pipe preparation.? For example, is it even possible to get down to white metal by sandblasting? What if the existing coating is asbestos, or lead paint? Is the area so isolated that a generator or powered equipment cannot be made available? What about UV exposure? Moisture? Extreme ambient temperature conditions? Extreme subsurface conditions such as rocks, erosion, soil conditions, next to railroad tracks, mechanical conditions due to soil impaction, unstable ground?
2. Preparation: To what NACE standard do I need to prepare the pipe for an appropriate coating? As mentioned above, is it even possible based on conditions to prepare the pipe to a particular NACE standard?
3. Experience of the crew applying the coating: This is a huge one and often the principle reason for coatings failure. Is the coating material so complex that an experienced crew must be on hand to do the application? Most often than not, the experience of the applications crew is lacking and for some of the more involved coatings procedures the crew contributes to coatings failure.
4. Performance: Clearly everyone wants the coating to last forever. Does the coatings manufacturer have a track record whereby they can exhibit actual field performance to support their coatings material? Do the actual coatings specifications match the external conditions? Unfortunately, there are examples where coatings manufacturer's have not run appropriate ASTM testing methods on the material they are recommending for certain applications. The pipe owner must be able to differentiate science from mythology.
There are other considerations, I have listed few
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