All That Shines is Not Always Stainless

For stainless steel fasteners, knowing which alloy to use above and below the waterline can save maintenance headaches.

These stainless-steel fasteners, when used below the waterline, can deteriorate from galvanic corrosion from lack of oxygen. In this case, the corrosion likely developed due to a combination of immersion and the use of a minimum quality stainless steel.

What’s with “stainless” steel fasteners that rust and leave a dark amber telltale stain?

We think of steel as strong, very strong, and we think of stainless steel as even more robust with a sparkling finish to complement its resistance to corrosion and rust. But all that’s shiny is not always the stainless steel you think it is. When it comes to the nuts and bolts of stainless steels used in marine fasteners (screws, nuts, bolts, washers), there are variations on the theme and there are some absolutes for marine applications.

Here’s the drill (pun intended): Bolts, nuts, washers, screws, hinges — virtually any hardware that can be fastened to a boat or its components — is available in several metal compositions, known as alloys.

Alloy steel is defined as steel with modified properties made by combining iron with one or more elements, in addition to carbon.

Alloys change the properties of the steel, for example, making it harder, more formable or more corrosion resistant, depending on the combination and amounts of alloys used. Stainless steels (alloys) encompass a very broad group of metals.

“Stainless steel” is a common term for describing steel alloys that consist of 10.5% or more chromium (Cr) and more than 50% iron (Fe). Although it’s called “stainless,” a better term for it is “highly stain resistant.” The chromium gives the steel its “stainless” properties, essentially corrosion resistance. On the surface of the metal, a very thin chromium-rich oxide layer, which is inert, is formed and prevents the steel from rusting. In general, the higher the proportion of chromium, the stronger the corrosion resistance of the steel. In addition to chromium, other metals are added to give the steel particular properties such as strength and malleability.

Stainless-steel fasteners are widely used in modern boatbuilding and repair because they provide strength and affordable corrosion resistance but only a few grades of stainless steel are suitable for boat fasteners. The 300 series stainless steels are the most popular. Among them are the 18-8 grades that contain very little carbon, about 18% chromium and 8% nickel, which give them good corrosion resistance. Type 316 series stainless contains molybdenum, which significantly increases corrosion resistance, strength and of course, cost. There are also “super” stainless steel alloys with significant additions of chromium, nickel, molybdenum or copper that should be used where requirements for extra corrosion protection, strength or heat resistance are paramount.

Other alloy types, namely 410, 302, 304 (18-8) and 305 stainless, generally provide excellent corrosion protection in most situations. However, in direct saltwater or other high chloride atmospheres these alloys may pit and discolor. This may cause color to bleed through finishes and lead to streaking in the wood around a nail or screw head.

Type 316 is the main stainless used in the marine environment, with the exception of fasteners. Where strength and wear resistance are needed then Type 304 (18-8) is typically used. Type 316 is recommended in saltwater and other highly corrosive exposures to prevent fastener pitting and/or head discoloration and possible bleeding through finishes.

What distinguishes Type 316 from Type 304 (18-8) is the addition of molybdenum up to a maximum of 3%. Molybdenum increases the corrosion resistance of this chromium-nickel alloy. As such, molybdenum is one of the single most useful alloying additives in the fight against corrosion.

Use stainless-steel fasteners cautiously below the waterline for they cannot survive in an anaerobic (starved of free oxygen) environment. Because oxygen is necessary for the reaction, immersion in water for a prolonged time prevents oxygen contact and promotes corrosion. If the screw is immersed in “still water” with no oxygen, the chromium oxide corrosion-resistant film cannot form. Without this film, fasteners soon suffer from galvanic corrosion and eventual failure.

Not all stainless steel is equal and you definitely get what you pay for. You just need to be sure you know what you need, why you need it and what you’re getting.
— By Patricia Kearns