Can you give us a description of where on the plate that this pin-holing is occurring? Is it occurring near a contact point (the point on the plate where it contacts an adjoining plate for structural stability) or away from contact (non supported) areas? Is it occurring near the ports? Top half/quarter of the plate, bottom, in the gasket trays, etc.
Give a basic description of the HX, two pass, four pass, more than that pass, simplex, duplex (two services in one frame) type? If it is mechanical and not corrosion, we need to know a lot more about the physical characteristics of the plate. What is the size of the heater, length and width of the plates.
Is there any significant temperature cycling?
Did you get any spare plates (or have any plates from the original plate pack that were pulled out for performance reasons) that you can look at to examine the area in question prior to being in service?
Sorry, but for me if it were chemical attack the chemical wouldn't be selective enough to pick this special point on each plate and ignore all the other crevices and corners and bends and twists and turns and even dead zones like gasket trays unless you had a phase change at the offending point. Right now I am going with mechanical defect or thinning due to rubbing at contact points. But then again, the jury hasn't come back in yet either until we hear from you more about his heater.
rmw