Ductile iron pipe (DIP) has become an industry standard that is used in many places within a water and wastewater treatment plant for several decades. Although DIP is strong, flexible and easy to “tap” into, it will still break down when placed into a corrosive environment unprotected.
The most common mistake I have seen in specifications is the use of The Society of Protective Coatings (SSPC) and NACE International abrasive blasting surface preparation standards when abrasive blasting Ductile Iron Pipe. The NAPF (National Association of Pipe Fabricators) 500-03 document it states “Attempts to apply steel surface preparation specifications to ductile iron pipe is inappropriate and may actually result in damage to the pipe surface with subsequent reduced coating effectiveness and life expectancy.”
One of the main criteria for determining if the SSPC/NACE abrasive surface preparations standards have been met is to “visually” inspect the surface. This “visual” inspection on ductile iron pipe, will show a much different appearance (turning a bluish grey due to the annealing oxide layer on the surface). This difference in color change may cause the contractor to continue blasting the DIP to achieve the “near white metal finish” but can cause the DIP to “sliver” and “bubble” from over-blasting. If these defects occur, the surface may be unsuitable for coatings to be applied to.
The NAPF has developed standards specifically for the surface preparation of ductile iron pipe. You will see the NAPF 500-03 standards closely mirror those of SSPC and NACE International:
- NAPF 500-03-01 Solvent Cleaning
- NAPF 500-03-02 Hand Tool Cleaning
- NAPF 500-03-03 Power Tool Cleaning
- NAPF 500-03-04 Abrasive Blast Cleaning for Ductile Iron Pipe
- NAPF 500-03-05 Abrasive Blast Cleaning for Cast Ductile Iron Fittings
More information on these standards can be found at www.napf.com.