engine to turn over the water in the stern tube and briefly spinning the shafts to change the portion resting in the bearings and seals will help re-oxygenate the stainless.
When the oxide surface film breaks down, the exposed metal loses its non-corrosive properties. The exposed metal will break down, forming a small hole or pit. This pitting tends to develop in areas where water flow is restricted, such as shaft seals and bearings. These happen to be areas hidden from view as well, keeping the pitting concealed until the shaft is pulled out.
At what point does pitting threaten the integrity of the shaft? That question poses a difficult judgment call. How deep are the pits and how many? Will the boat be inland or offshore? What is the budget? Generally, pitting less than 1/8-inch deep can be tolerated, but that might be trumped by an excessive area of pitting. A process known as “cladding” can restore a pitted shaft. Cladding involves machining, welding, more machining, and straightening, and must be done by a highly trained machinist. Due to the expense of cladding, it usually is not cost-effective for shafts under 3 inches in diameter. In addition to strength concerns, pitting in the bearing area or the shaft seal can damage the bearing or seal.
What can you do: When buying a new shaft, go with 22 grade if your budget allows. If your current shaft attracts a magnet, be on the lookout for pitting corrosion at your next haul-out. If preparing for a cruise, pull the shaft first and fully inspect.
Although the chemical process for crevice corrosion does not differ from that of pitting corrosion, the causes vary. Crevice corrosion refers to a localized attack at the interface between two surfaces, or where a sharp edge occurs. This situation can occur, for example, where the forward end of the propeller hub meets the shaft. On lower-grade alloys this area will suffer from severe crevice corrosion (see below). On a microscopic scale, the same process takes place where small scars or nicks in the shaft surface develop. These small imperfections create flaws in that critical oxygen film, providing a foothold for crevice corrosion. One study published in the Journal of Failure Analysis and Prevention
looked into the failure of a 2-inch diameter shaft that failed near the forward end of the taper. Relying on sophisticated testing techniques, they concluded that rotational fatigue began from the surface of the shaft near the keyway. They determined that the origin of the failure was “located on a surface flaw (recess or dent) of approximately 100 microns in depth.” That is merely one-tenth of a millimeter. While it might seem that a shaft fails all at once, in fact it is a process that begins long before the break. Microfractures form first, eventually leading to a small crack that grows until the shaft strength no longer exceeds the working loads.
What you can do: Never use a pipe wrench or pliers on your shaft. Avoid any- thing that would scar the metal. Barnacles can also provide starting places. Keep the running gear clean. Consider using one of the products designed to prevent marine growth on metals. Cleaning a shaft with a steel brush can leave surface contamination that invites corrosion.
Whenever you have your prop pulled, your shaft removed, or prior to any extended offshore voyaging, request a liquid penetrant test in the highest stress areas, around the keyway and just forward of the propeller hub. When cracks appear, the shaft should be replaced.
Finally, we turn our attention to galvanic corrosion. Galvanic corrosion occurs when dissimilar metals are immersed in a conductive solution (seawater). Regardless of the alloy, your shaft needs a sacrificial anode. A collar anode attached directly to the shaft provides the answer. If your boat has a large anode on the transom connected to the bonding system, it will protect all bonded underwater metals. The shaft and propeller cannot be assumed to be bonded through the engine. The shaft connects to the transmission, but the electrical continuity through the oil-bathed gears and bearings does not suffice. Bonding circuits must have less than 1 ohm of resistance. For this reason, a shaft brush is required to properly connect the shaft and prop to the common anode.
Shaft failures in commercial shipping have prompted numerous studies and in-depth analyses. The forces and chemistry at work here reach an astounding level of complexity, with many interwoven processes. Instead of focusing on any one detail, a comprehensive approach provides the best results. Start with the proper shaft alloy, preferably 22 grade, make sure the propeller has been properly installed and meets the basic parameters described above, handle the shaft with care to keep it free of imperfections, and provide appropriate sacrificial anodes. Oh, and most important, keep a healthy flow of oxygen coming—that is, use your boat often.