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I have noticed during the servicing of ScubaPro Mk15 first stages (its not like I have enough to do with my primary job) there seems to be a number of pistons that are pitted, often deeply, in the o-ring groove. The Mk15 uses a stainless steel piston, at least thats what I have been told by ScubaPro. There are several grades of stainless steel available, some of which are not very suitable for use in seawater. ScubaPro will not tell me what grade they are using, but they have admitted there can be pitting in this groove. Its interesting that one way to instigate and accelerate the onset of crevice corrosion in stainless, for material testing purposes, is to use a tight-fitting o-ring to form the initial crevice (a practice I learned from the LaQue Center for Corrosion Control). ScubaPro did indicate they would replace pistons returned to them with pitting. Water flushes into this groove through the ambient pressure reference ports and may be further entrained in with the grease in the groove. How do you minimize the potential problem? Seal off your first stage inlet port and soak your first stage for a couple of minutes in freshwater after use in salt water (my opinion is that saltwater is the main culprit). Have the regulator serviced regularly and have the technicial pay particular attention to the piston. If he/she cleans and lubricates the piston properly it will probably not be a problem. I have yet to see a catastrophic failure (doesn't mean it can't or has not happened) and the pistons still seem to seal even with the pitting (the actual seal is not in the base of the groove where the pitting is generally located). If you use the Mk15 I would not panic, just pay a little closer attention to preventive maintenance. Just my opinion, Doug Chapman
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