Stainless steel is the most widely-used metal for fittings and fastenings on boats. Although it shares family linkage to your boat's engine block and the plating on steel boats (they're all ferrous alloys), the strengths and weaknesses of stainless steel requires some scrutiny. There are three sub-groupings of stainless steel:
What is Marine Stainless Steel?
Marine stainless is composed primarily of iron (at least 50%), with chromium (10%) and nickel added. Once exposed to the elements, the chromium enables the alloy to form a tough oxide film and thus gives stainless steel its unique resistance to rust. Marine grade lies within the 300 series and for most boating applications, that means grade 304 and 316. The difference between these two is the proportions of other elements added to the basic iron.
Grade 304. Is generally used for fasteners, rigging and fittings. It is sometimes referred to as 18-8 because it's made by adding 18% chromium and 8% nickel.
Grade. 316.contains more nickel than 304, and 2% molybdenum. It is the most corrosion-resistant of standard stainless steels, is more expensive, and has about 85% the strength of grade 304.
Finding increasing use in marine work, particularly for forming and bending, is Grade 445M2, which has equal corrosion resistance to gr. 316 but costs less.

Weak Points
1. Crevice Corrosion. Stainless steel normally remains corrosion free because the chromium in it oxidises (reacts with the oxygen in the atmosphere) to form an inert skin that protects the metal. If stainless steel is deprived of oxygen a destructive process called crevice corrosion will soon begin and it can occur above the water line. Rust stains are a sure sign. Stainless fastenings holding sea-cocks, struts or shaft logs should be closely watched. Common problem areas are anywhere there is wet wood or fibreglass, or there are stainless plumbing fittings that hold or deliver sea water.
2. Welding Stainless Steel.
Weld migration, weld decay or carbide precipitation are all terms describing failure when stainless steel is welded. It occurs when heat drives certain elements out of the alloy adjacent to the weld bead, leaving
a 1-2 mm, strip of mild steel along either side of the weld. Obviously a weld is only as strong as the metal surrounding it, so be cautious when welding stainless of unknown stock. Suitable product for welding should include the suffix 'L' (e.g. 304 L or 316 Q meaning it has low carbon content.

Guarding against crevice corrosion within PVC jacketed stainless steel lifeline is an important safety consideration. Brown staining around the swaged end fitting is a classic sign that water has migrated between the wire and the plastic cover. Once there in this oxygen depleted area, corrosion sets in quickly by breaking down the oxide film on the wire and swaged fitting. In this state, the integrity of the boat's entire lifeline system is questionable. This is why the Australian Yachting Federation has banned PVC jacketed lifelines.

Read more --> | Stainless Steel | Nickel Institute | Australian Stainless Steel Development Assn. |

Marine Metals and Galvanic Corrosion. In 1780, Luigi Galvani first recorded the effect of current flow caused by the contact of dissimilar metals in an electrolyte. Yet even now, metal components in the marine environment is a subject that is poorly understood and often mishandled.
An electro-chemical reaction takes place whenever different metals are in contact, either electrically or mechanically, and are immersed in a conductive solution such as sea water. That means that an electric current will flow, resulting in corrosion of the least noble, or active metal. In an electro-chemical reaction, this is the electrically positive anode. The most resistant, or passive metal of the two is the electrically negative cathode, and it usually corrodes very little or not at all.

Galvanic Series for Metals in Seawater.

Noble. (Cathodic. Electrically negative. Least corroded.)

Active. (Anodic. Electrically positive. Most corroded.)

What Does the Galvanic Table Tell Me?
Simply put, metals higher in the table may cause increased corrosion to those lower in the table. Where metals are close together in the table there is usually little adverse effect. Particularly undesirable couples are therefore copper, aluminium alloys with aluminium, and steel with aluminium.
Although the driving force for corrosion is the position of metals on the galvanic table - that is, their potential difference - it is the actual current density which determines the rate of corrosion. Consequently, large cathode/small anode combinations are particularly serious and large anode/small cathode combinations are usually acceptable. For example, steel fasteners on copper alloy components in seawater would corrode very quickly.

The two positions that stainless steel occupies on the above table illustrates how oxygen depletion affects its resistance to corrosion. The normal passive condition is more cathodic (noble) than most other metals. However, when the metal is in an oxygen deprived environment (crevice conditions), the less noble/active condition prevails.

A single metal fitting, immersed in seawater, that is not connected to another more noble metal, can still have galvanic corrosion occur. This is because an alloy is, by definition, a mixture of metals. Electrical current flows internally within the fitting, because there is a potential difference in the component elements. For instance, brass is comprised of zinc and the nobler metal, copper. The zinc dissolves, leaving a spongy copper residue. This is why underwater metal skin fittings and valves should be chosen carefully. Bronze (copper and tin - more corrosion resistant) is always preferable to brass (less corrosion resistant), but they can be difficult to tell apart. Bronze fittings are almost always cast - they are rough in feel and appearance. Brass on the other hand, has a smooth interior and exterior, because they are drawn through a die when they are made.

Sacrificial Zinc Anodes.
Ideally a boat should be constructed so that the metal components are compatible - that is, they are close on the galvanic table. Stray currents from within and outside the boat must also be considered. These corrosion sources are called electrolytic and are typically caused by poor or faulty wiring aboard, shore power into the boat, or suspect power management on a neighbouring boat. So, sacrificial zinc anodes are probably necessary. Placing these on your boat's hull, rudder, propeller-shaft, engine water cooling system and refrigeration condenser will significantly extend their life. The current flow between the zinc anode and the hull/fittings - the cathode - will see the anode corrode proportional to the level of current flow present. In other words, the greater the imbalance (the potential difference), the faster the zinc anode will corrode.

Unpainted aluminium hulls are susceptible to intermatallic corrosion if lead sinkers or brass fittings are left in contact with the hull in the presence of salt water, resulting in perforation of the hull.

For further information on corrosion protection, visit | Industrial Galvanizers | Antifouling 1 | Antifouling 2 |

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