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10月 2018

Tips to Avoid Stainless Steel Corrosion on Boats

cleat marine hardware-vast casting

cleat marine hardware-vast casting

Bright, shiny stainless steel fixtures on sail or power boats — from cleats and fishing rod holders, to mast shrouds and winches — can really make a vessel “pop” and serve their purpose well if maintained properly.

Of course, boat owners have a more vested interest in caring for onboard stainless steel equipment, but boat charterers, particularly on voyages of more than a few days in a saltwater environment, should also know how to maintain and care for stainless steel fixtures because the material requires constant, but, fortunately, simple care.

A short and non-scientific discussion of stainless steel metallurgy, presented below, along with some tips on caring for stainless steel, can help effectively maintain stainless steel equipment, fixtures, and fittings on board a vessel.

What is Stainless Steel and How Does it Work?

Stainless steel is an amalgam of metals that contains more than 10% chromium. Marine grade stainless steel (known by its industry number, Type 316) also contains amounts of molybdenum, nickel, and nitrogen. All four elements work together to make stainless steel the metal of choice for a variety of applications in harsh marine environments, especially saltwater. Stainless steel resists corrosion (called pitting), maintains its strength at high temperatures, looks good, and is easily maintained. Stainless steel differs from carbon steel by the amount of chromium content in the metal.

While stainless steel is corrosion resistant, it is not corrosion proof. When exposed to harsh air and moisture conditions, especially in tropical climates and around saltwater, stainless steel corrodes or rusts (also called oxidation), which creates iron oxide. The chromium in stainless steel inhibits this process by producing its own oxide, chromium oxide, a rough, corrosion-resisting microscopic film on the steel surface that resists further oxidation. In freshwater environments, stainless steel corrosion still occurs, but at a much slower rate.

Types of Corrosion That Affect Stainless Steel on Boats

  • Atmospheric: Occurs when stainless steel is exposed to airborne liquids, solids, or gases, such as sea spray, rain, salt, or dirt.
  • Chemical: Occurs when the steel comes in direct contact with a material with high chloride or corrosive materials, like bleach.
  • Contact: Occurs when small (even microscopic) pieces of carbon steel, scale, copper, or other foreign material become lodged in stainless steel, causing pitting.

Tips for Caring for Stainless Steel on Boats

  • Use a soft cloth, like chamois, or a non-abrasive sponge together with fresh water and a mild cleaner, like dish detergent, to clean stainless steel fixtures. The same material can remove pitting. Steel wire brushes, sandpaper, or steel wool should never be used.
  • Marine grade stainless steel fittings/fixtures are vastly different from chrome-plated equipment. It pays to know what equipment on board is plated in order to maintain the equipment properly.
  • Naval jelly (also known as phosphoric acid) is a mineral-based, inorganic compound that can be used as a rust inhibitor on stainless steel.
  • Stainless steel is non-magnetic; applying a good magnet to onboard metals will determine the type of material.
  • After each saltwater voyage, regardless of length, a thorough fresh-water hosing of the deck and all stainless steel fittings, together with some robust Norwegian steam (elbow grease to landlubbers), will dramatically reduce the potential for corrosion.
  • Regular cleaning and making immediate repairs to damaged stainless steel safety equipment, such as stanchions, rails, handholds, etc. are critical.
  • A polish coating applied regularly adds protection and shine.
  • Because saltwater chlorides attack the chromium oxide film that inherently forms on stainless steel, cleaning products containing chlorides or acids, especially bleach, should not be used.
  • If warm water and mild detergent do not remove steel stains, a paste of baking soda and warm water works well. If the stain persists, add vinegar to the paste to boost its strength and add pressure while rubbing.
  • A halo may form around a rust spot on stainless steel, which can indicate that a different type of steel (like steel wool or steel brush strands) are embedded in the surface. A paste solution of 10% nitric acid and 2% hydrofluoric acid (also called pickling paste will remedy the situation. After applying the paste, rinse with a solution of baking soda and water, which allows the protective chromium oxide film to reform.

What is Electrolysis

The electrolysis phenomena is an electro-chemical process that occurs when two dissimilar metals come in contact with each other in a saltwater environment. For example, when aluminum and stainless steel are in contact, the chloride in saltwater or salty air creates an electrical charge that, if not controlled, causes corrosion to the involved metal lower on the galvanic scale (in scientific terms, the less noble metal).

Cathodic protection is the most common technique used to inhibit corrosion on dissimilar metals. In its simplest form, the technique involves attaching a sacrificial anode, a material with a more negative electrode potential, like zinc, aluminum, or magnesium, which takes the brunt of the formed electrical charge. The sacrificial anode eventually corrodes away and needs replacing. Protection can also be provided by connecting an electric power supply to oppose corrosive galvanic currents. Boat manufacturers go to great lengths to design and build vessels that minimize the use of dissimilar metals that contact each other, but there are areas on a boat where the contact is unavoidable, like lower units.

  • Know the areas where dissimilar metals are in contact and take steps to ensure that cathodic protection material (sacrificial anodes or electric equipment) is in good shape and replace or repair when needed.
  • Care should also be taken if affixing new metal fixtures, fittings, or other equipment to in-place metal materials to avoid using dissimilar metals, if possible.