Summary for bronze sand castings chlorine Discussion
Effects of chlorine on bronze sand castings are discussed in the context of a bronze part that was not nickel and chrome plated properly and then submerged in chlorinated water. Thomas Chianese asked what would happen, and the answer warned that a deficient coating can be worse than no coating at all: a small exposed anodic spot in the bronze beneath a large nickel-chrome cathodic area can greatly accelerate corrosion at the defect. The response also noted that pits, cracks, scratches, or other exposed base metal areas can become corrosion hot spots, especially because pits can trap corrosive plating solutions. As a practical remedy, the answer recommended vacuum impregnation after machining to seal pits, and pointed to MIL-STD-276 for a brief discussion of impregnation. The overall message is that poor plating quality and porosity can make chlorinated-water exposure much more damaging than expected.
Q. If a bronze sand casting is not nickel & chrome plated properly, what would be the effects if the casting was submerged in chlorinated water?
Thomas Chianese
castings – Mt. Vernon, New York
2003
A. From my old days in Corrosion 101, sometimes coating an active metal (i.e. anode)with a deficient passive layer (i.e. cathode) is worse than leaving the part uncoated! Small anodic areas, such as a pin hole of bronze (copper-tin) exposed through large cathodic area (nickel-chrome) greatly accelerates the corrosion in the area of the pit (or crack, scratch, or other means of exposing base metal). One obvious suggestion, assuming that pits in the casting may be the cause of the corrosion is to vacuum impregnate the castings following machining to seal the pits. Pits are notorious for leading to problems, especially after plating since they adsorb corrosive plating solutions which in themselves can initiate and accelerate corrosion. Contact my firm if you wish to further explore the possibility of resin impregnation of the castings. Consult MIL-STD-276 for a brief discussion of impregnation.
Milt Stevenson, Jr.
Plating shop technical manager – Syracuse, New York
2003
Q. I am considering to have work performed that includes replacing a “nickel-resist” shaft end-seal component with a similarly designed bronze material. The bronze material is of a non-contact design that may be submerged in water most of the time while in service. This water has been tested and resulting in containing chloride counts at approximately 125 ppm. Discoloring is expected.
Would the bronze material be suited for this type of application?
Martin Castro
water reclamation district – lockport, illinois, usa
July 27, 2012