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Fastener Metals


Marine Fasteners and Corrosion

The restoration of these wooden antiques is an expensive, time-consuming affair. If you want it to last, use the best materials available. The fasteners that hold the wooden components together are an important factor in this endeavor. WoodenBoat Magazine, Issue 55, has a great article on fastener corrosion by Ed McClave. Some of the following information is gleaned from the Book of Fastener Standards, 6th Edition, from the Industrial Fastener Institute.

The original brass fasteners should not be used, as the zinc component in the brass alloy (60 percent copper, 40 percent zinc) has shown to not last very long due to electrolytic corrosion, a process known as dezincification. Wooden boat manufacturers like Chris-Craft knew that nonferrous fasteners (contain no iron or steel) should be used, and by 1926 were proud to announce this in their advertising. Chris-Craft and others switched to bronze fasteners in the 1950's, and silicon bronze is the industry standard for wood fasteners in marine use to this day.

 
Some restorers have been lured into using stainless steel fasteners. These are harder than bronze and require less care in drilling the pilot holes, hence, they are less expensive from a labor stand point. Being made in the Far East, stainless steel fasteners are also marginally less expensive to obtain. However, these fasteners are still about 75 percent steel. Even 70 years ago Chris-Craft knew better than to use steel in their hulls below the waterline. (They and other builders did use stainless steel and chrome-plated fasteners above the waterline in trim items like cutwaters and rub rails.)
Stainless steel is an alloy of steel and a number of other elements that, in the constant presence of oxygen, maintain a surface treatment (passivation) that protects the steel from corrosion. Because trim items above the waterline are constantly in direct contact with oxygen, and revert from passive to active, leaving you with a steel fastener subject to quick corrosion in a marine atmosphere. Without oxygen, you may as well just drop the "stainless" adjective. The corrosion process of steel, better known as rust, also produces mineral salts that destroy the surrounding wood, seen as a black stain referred to as "iron Sickness." 

 The stainless steel available for the boat restoration industry is from the family of steel referred to as 18-8. This group consists of #302, 302HQ, 303, 304, 304L, 305, 308, 309, among others. Each number signifies minute differences in the makeup of the alloy. All have varying amounts  of chromium (usually 18 percent) and nickel (usually 8 percent) that create the surface repair oxidation, chromium oxide, which still requires oxygen to maintain. These are still roughly 75 percent steel.

For example: The lowest grade of stainless is  #302 made of 17 to 19 percent chromium, 8 to 10 percent nickel, and .15 percent carbon. This leaves roughly 75 percent steel. Only slightly higher is the #304 grade. Its chemical makeup is slightly different at 18 to 20 percent chromium, 8 to 10 percent nickel, and .08 percent carbon. They all differ slightly like this, but each grade is made to provide a different surface corrosion oxide layer. The higher the grade, the more expensive.

Of the stainless steel bolts and screws available to the boat restorers, 99.9 percent are made of the lowest grade available, #302. Actually, the alloy used for these is #302HQ, which includes a minute surface layer of copper to prevent surface fractures during the cold-stamping operations of head-forming and thread-rolling. Washers are stamped from #303, and something is added to make that stamping operation run cleaner and smoother. If your restorer claims that he's using "high grade" stainless steel in your boat, ask him to clarify that. He may be kidding more than himself.

Some suppliers are beginning to print disclaimers in their catalogs. Look in the front of your Jamestown Distributors catalog. Under the heading "Technical Fastener Details" (under Stainless Steel) , they state: "Use stainless steel Cautiously below the waterline. Stainless steel cannot be used in an anaerobic environment (this means no air or free oxygen). If the screw is immersed in 'still water' with no oxygen, the corrosion-resistant film, chromium oxide, will not be allowed to form. Without the chromium oxide film, the screw will suffer from galvanic corrosion and eventual failure."

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