Saltwater, Fasteners, and Boats
Marine Fasteners is sometimes consulted by boat builders, who are receiving complaints from customers about corrosion appearing on some fasteners in their boat. The causes for this can be infinite, but we will attempt to explain some of the more common problems below, as briefly as possible. All metals except gold, platinum, and palladium will corrode. Stainless steel becomes rust resistant with the addition of chromium. This chromium, when coming in contact with oxygen, forms a 130 Angstroms thick (angstrom =one millionth of one centimeter) natural barrier of chromium oxide called a "passive film". For this passivation to properly form, a fastener must be cleaned (passivated) in nitric or citric acid, which removes production oils, metal and tooling fragments. According to the Department of Defense, there are eight major types of corrosion, not all of which will affect stainless steel fasteners. They are uniform attack, crevice corrosion, pitting, galvanic corrosion, intergranular corrosion, selective leaching, erosion corrosion, and stress corrosion. We primarily will deal with crevice, uniform and galvanic corrosion when discussing possible corrosion on fasteners and boats in seawater. Crevice corrosion is simply the interference of the stainless steel fasteners ability to self-passivate itself. This can begin with the fastener being improperly cleaned (passivation) after manufacturing. But even a correctly passivated part can experience crevice corrosion when a fastener is "starved" of oxygen. This is especially true with stainless in low PH seawater, but can also be a problem with polluted or brackish fresh water. This corrosion is primarily found in oxygen starved areas, such as under fastener heads, or inside screw drives. Uniform corrosion is the attack on the passive film by a halogen. Halogen's can be recognized easily, because they end with "-ine". The chlorides from the salt in seawater, attack and destroy the passive film faster than it can reform in low oxygen areas. Chlorine, when in contact with stainless, attacks the passivation of stainless steel and will usually find its weakest point of resistance (see crevice corrosion). Once it breaks thru the passivation layer, it begins to attack the fastener itself. Marine Fasteners provides many boat builders chrome-plated stainless steel fasteners. This provides an extra barrier to salt-water intrusion, adding a delaying barrier to the chlorine attack (see salt spray test results here). (This plating process also follows the seawater rule of thumb - "paint the cathode". Dr. Harvey P. Hack, Northrop Grumman Corp., wrote in UnderWater Magazine Summer 1999 "One of the best 300-series stainless steels is type 316. Even this alloy will, if unprotected, start corroding under soft washers, in o-ring grooves, or any other tight crevice area in as little as one day, and it is not unusual to have penetration of a tenth of an inch in a crevice area after only 30 days in seawater.") Galvanic corrosion is the placing of two dissimilar metals in a electrolyte, producing an electrical current. The current flows from the anodic metal and towards the cathodic metal, and in the process slowly removes material from the anodic metal. Water makes a good electrolyte (electrically conductive), seawater even better. Thus, galvanic corrosion is a constant variable in proper boat building design. In aluminum boats, the area of the cathode (stainless steel screw, the noble metal) is very small, and the anode (aluminum, the active metal) is very large, the current produced will be very low and the anode will corrode very slowly, if at all (SSINA). The ratio is important as an aluminum rivet (anode) used on a larger stainless steel surface (cathode), would cause galvanic corrosion in the aluminum rivet. While impractical in many applications on a boat, a plastic sleeve and washers separating the fastener from the base material can remove any potential for galvanic corrosion. While you, as the boat owner, are counting on the stainless steel fasteners in your boat not to corrode, the simple fact is, they won't - with your help. Boat-builders can not afford to build a boat with corrosion-"proof" fasteners, such as gold or platinum. However, they can and many do design them, using "corrosion-resistant" fasteners. How they are treated after they have left the dealer showroom, is beyond their control. So what can you do to help extend the life of the fasteners on your boat? #1 Most importantly, keep your boat clean. Wash it with clean fresh water after you have been out to sea, and often, when you haven't. A boat sitting in seawater is a much easier target for corrosion, than one constantly moving. The spray from seawater, allowed to settle, adversely affects stainless steel miles inland. You can imagine how a boat is being affected when it sits in seawater 24/7. The chloride-containing saltwater must be washed off the stainless steel surface and the stainless steel be allowed to re-passivate, or you will start seeing corrosion almost immediately. Chrome plated stainless steel screws, while delaying this attack, are not a magic bullet, and will eventually succumb to the chloride attack. #2 Make sure your anodes, or as some call them in the marine industry, the "zincs", are working properly. The anode is added to your boat to "sacrifice" itself as part of the natural galvanic corrosion that will happen in seawater. If your anodes are not corroding, you could have a bigger problem somewhere else in the boat. Ask your dealer to point out where the sacrificial anodes are on your boat. If they are working properly, they will start showing corrosion soon after being put in the water. #3 When plugging a boat into shorepower, make sure you use a galvanic isolator. While some would classify this need as part of your protection against galvanic corrosion, others give this specific type of corrosion its own name - "stray current corrosion". Stray current corrosion is taking the electrical potential of dissimilar metals, as described above in galvanic corrosion, with "stray" electricity added to the equation. Stray electricity, from an outside source, enters your boat thru its metal components, and back out in search for a ground. The source could be as simple as a boat sitting next to you leaking DC current, and the way out of your boat could be your shorepower connection. The galvanic isolator can help protect you by disrupting this 'current' flow, and potentially make your boat a greater path of resistance, causing the stray current to look beyond your boat for its ground. #4 Check your electrical system to ensure you are not leaking DC current. #5 And finally, did we mention how important it is to clean your boat with fresh water, often?
Sources
http://www.ocean.udel.edu/seagrant/publications/corrosion.html |