Article by Capt. Steve Katz
Heavy Metal
When many of us think about fishing boats, we think of shiny fiberglass or paint and powerful engines. While various metals are part of every boat, metal is not often thought about until there is a problem.
Look around any boat and the first metal parts you often see are the screws, nuts, bolts or other fasteners that help hold it all together. Dig a little deeper and there are various metals that make up plumbing systems, engine cooling systems and many other critical parts of a vessel. Most marine metals are designed for exposure to the marine environments, such as sun, hot temperatures, cold temperatures fresh water, salt water etc. While most metal parts are up to the task of enduring the environments, some need special selection and attention when it comes to the salt water environment.
Stainless Steel
Stainless steel is an alloy of steel. There are many variations, but they all contains at least 50% iron and 10% chromium. Other elements are added to the basic alloy to create variations of stainless steel for specific applications. Marine grade stainless steel is most often found in the form of a 300 series alloy. The most common alloys used in recreational marine fasteners are 304 and 316. What’s the difference? The 304 alloy contains 18% chromium and 8% nickel while the 316 alloy contains 16% chromium, 10% nickel and 2% molybdenum. The molybdenum is added to help resist corrosion to chlorides such as seawater. Unfortunately for consumers, there is no way to visually tell the difference of the variations of stainless steel. You will have to rely on the manufacturers information or send the metal to a test lab for analysis.
Why doesn’t stainless steel often corrode? The chromium in the stainless steel combines with oxygen in the environment to form an invisible surface layer of chromium oxide that prevents corrosion from spreading into the metal's internal structure. This chromium oxide layer will automatically self-repair if damaged. If there is a lack of oxygen at the fasteners surface, the chromium oxide will not form, and corrosion will begin. When it comes to marine fasteners, corrosion can begin where a stainless steel fastener is surrounded by a moist environment, such as a through bolt for a deck fitting where the deck has a high moisture content. The oxygen in the waterlogged deck gets depleted trying to form chromium oxide so with no more oxygen the corrosion begins. Therefore, it’s important to seal openings, such as screw holes in deck hardware, when using stainless steel fasteners to prevent a damp environment where there is lack of oxygen. This is referred to as crevice corrosion and is one of the 8 recognized forms of corrosion.
Bronze, Copper and Brass
Bronze, copper and brass are often thought of similarly and are referred to as “red metals”. They are all often found on a boat in various applications, but boaters need to be cautious of their use and be sure not to mix them together.
Bronze
Bronze is an alloy consisting primarily of copper, with about 12% tin and often other metals such as aluminum, manganese, nickel or zinc. The combination of these ingredients produces an alloy much harder than copper alone.
A popular specialty bronze alloys on a boat is referred to by the alloy elements - Nibral (nickel, bronze and aluminum), Nibral is often used to make propellers. It is a strong and corrosion resistant alloy. Silicon bronze is comprised of 96% copper with silicone and other elements, is a popular alloy used for underwater metals, such as seacocks, struts, nuts and bolts.
Copper
Copper is a naturally occurring metal, it is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. Copper often has a reddish-pinkish-orange color in its natural state. Pure copper isn’t often used for marine parts since copper alloys are stronger and more corrosion resistant that pure copper.
Brass
Brass is an alloy of copper and zinc and often other metals added for specific applications. Brass is often found in above the water line plumbing fittings on boats and used for decorative purposes.
Different types of metals should not be mixed together in a system where direct contact is made. When it comes to plumbing fittings such as Bronze and Brass, they are often sold in the same part of the supply store and are often difficult to tell apart from each other. These two different materials will galvanically corrode when immersed in an electrolyte, such as saltwater, leading to a catastrophic failure.
Dezincification
Brass fittings work well in some applications, such as a domestic water system, but improperly used in a salt water system, or in any below the water line plumbing, can lead to failures. Due to the its high zinc content, brass can suffer from corrosion known as dezincification; when this happens, the zinc sacrificially corrodes from the alloy, leaving behind a porous, weak copper structure. The remaining structure may look okay, but is extremely weak in strength compared to the original brass. When dezincification occurs, the surface is often notably pinkish or reddish in color, which is the raw soft copper.
Galvanic corrosion
Galvanic corrosion is an electrochemical process in which one metal corrodes when it is in electrical contact with another metal in the presence of an electrolyte (salt water). This is the most common type of metal corrosion found on pleasure boats and is usually contained to the underwater metals. A proper bonding system and sacrificial anodes (commonly referred to as a Zinc, though it can be made from other metals) are the best way to control galvanic corrosion.
Choosing and caring for the metal parts on your boat is a critical part of boat ownership. Learn to know what metals are best for each application and be sure that they are not corroding or failing in some way due to the harsh marine environment.
Captain Steve Katz is the owner of Steve’s Marine Service and holds NMEA, AMEI and NMEA2000 certificates along with ABYC Master Technician certification and factory training from many manufacturers. To reach Steve, call 410-231-3191.
