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2nd May 2012

Dufour Association

Technical - Page 1

Grades of Stainless Steel screws etc for marine use.

Fastener Materials and Coatings Information
(From www.Fastfixdirect.co.uk)

Fastener and Fixing Materials
General

Fasteners and fixings such as  hexagon nuts, hexagon bolts, coach bolts, blind rivets, solid rivets, woodscrews, machine screw, nails etc, are manufactured in a wide range of materials from common steel to  titanium, plastic and other exotic materials. Many materials are further separated into different grades to describe specific alloy mixtures,  hardening processes, etc. In addition, some materials are available with a variety of coatings or plating to enhance the corrosion resistance, or appearance of the fastener.

Fastener material can be important when choosing a fastener due to differences between materials  in strength, brittleness, corrosion resistance, galvanic corrosion properties, and of course cost.

When replacing fasteners, it is generally best to match what you are replacing. Replacing a bolt with a stronger one is not always safe. Harder bolts tend to be more brittle and may fail in specific applications. Also some equipment is designed so that the bolts will fail before more expensive or critical items are damaged. In some environments such as salt water galvanic  corrosion must also be considered if changing fastener materials.

Materials

Stainless Steel
Stainless steel is an alloy of low carbon steel and chromium for enhanced corrosion characteristics. Stainless steel is highly corrosion  resistant for the price and because the anti-corrosive properties are  inherent to the metal, it will not loose this resistance if scratched during installation or use.

It is a common misconception that stainless steel is stronger than regular steel. In fact, due to  the low carbon content, stainless steel can not be hardened. Therefore when compared with regular steel it is stronger than mild steel fastener but weaker than hardened steel fasteners.

Stainless steel is also much less magnetic than regular steel fasteners though some grades will be slightly magnetic.

Good corrosion resistance is a feature of all stainless steels. Low  alloy grades can resist corrosion in normal conditions. Higher alloys  resist corrosion by most acids, alkaline solutions and chloride environments.

The  corrosion resistance of stainless steels is due to their chromium content. In general, stainless steels contain a minimum of around 10.5% chromium. The chromium in the alloy forms a self-healing protective  clear oxide layer that forms spontaneously in air. The self healing nature of the oxide layer means the corrosion resistance remains intact  regardless of fabrication methods. Even if the material surface is cut or damaged, it will self heal and corrosion resistance will be maintained.  The most commonly used grades are A2 stainless steel and A4 stainless steel, these are the two types offered by Fast Fix Direct Ltd.

A1 = a free-cutting quality, having a superior machine ability due to a higher  phosphorus and sulphur percentage. As a consequence, however, the general corrosion resistance is decreased. This "automatic lathe" stainless steel is seldom used for mass production fasteners.

A2 = the most commonly used stainless steel grade - also called 18/8 (18% Cr, 8%  Ni) - with outstanding corrosion resistance under normal atmospheric  conditions, in wet surroundings, oxidising and organic acids, many  alkali and salt solutions.

A4 = the most corrosion resistant steel grade - also called "acid proof" - with an increased nickel percentage and addition of molybdenum. Better resistance to aggressive, corrosive environments such as sea climate (chlorides), industrial atmosphere (sulphur dioxide), oxidising acids and there where pitting may occur.

Stainless Steel fasteners are the work horse for modern boat-building or exterior woodworking applications. They are perfect for marine use due to great corrosion resistance provide while  remaining affordable. Hardened steel screws are more common in the woodworking environment but cannot be used in a corrosive environment. Stainless steel is available in a wide range of engineered grades, each  with its own designation number and specific properties. Only a few grades of Stainless Steel are suitable for fasteners, those that can be  cold headed easily and have adequate torsional and shear strengths. The 300 series stainless steels are the most popular.
The most commonly used grade is A2 (304), however A4 (316) is a major step up and recommended for use in highly corrosive  environments, such as in contact with salt water. A4 (316) series stainless, also known as marine quality or food grade stainless  steel, contains molybdenum, which significantly increases corrosion  resistance and strength.

Use Stainless Steel screws cautiously below the waterline. Stainless Steel Screws cannot be in an anaerobic environment. 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.

When using stainless steel nuts and  bolts especially in larger in size always remember to lubricate with an  ant seize compound. This will prevent the nut from binding on the thread (known as galling or cold-welding).

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