What Stainless Steel Grade Should I Use?

There are several considerations when choosing which grade of stainless steel.

Choosing the right grade – how corrosive is the environment?

Grade 304 stainless steel is a suitable grade for non-corrosive environments. The Australian Stainless Steel Development Association recommends that grade 316 stainless should be used as a minimum within 5 kilometres of the beach or sea.

Duplex grade 2205 stainless steel is suitable in and around salt water and other highly corrosive environments, but Super Duplex 2507 or equivalents such as 6MP and 254MO are necessary if permanently submerged in salt-water. The corrosion resistance of each grade of stainless is described by its PREN Value which will often determine which grade is required.

Choosing the right grade of stainless to prevent galvanic corrosion

Any two dissimilar metals in contact with an electrolyte present (such as salt-water) have the potential to create a galvanic cell and cause galvanic corrosion. Such an electric cell will cause the more anodic of the 2 metals to be attacked by galvanic corrosion. Or put better, to be corroded at an accelerated pace by galvanic means.

Ideally dissimilar metals should always be separated and/or insulated from each other, including stainless steel in contact with other metals. To avoid galvanic corrosion though there are other points to consider which may negate the need, such as:

• The lack of an electrolyte fluid present (I.e. a dry location with no water).
• The proximity of the 2 metal on the galvanic series (I.e. Grade 304 and 316 are so close on the galvanic series that galvanic action is highly unlikely).
• The surface area of the cathode vs anode (I.e. a small stainless steel screw on a large aluminium window frame will create less of a problem than a small aluminium screw in a large stainless steel window frame. Size does matter!)

Choosing the right grade of stainless based on cost

Although stainless steel is more expensive than carbon steel in most cases, often the overall life cycle cost of the stainless will be lower. This is due to the reduced in cost of replacement and re-work over time.

Grade 316 stainless steel can be anywhere from 25-80% more expensive than other grades of stainless (such as 304). this varies as the price of Molybdenum, Nickel and Chromium vary over time, but the cost is usually around a 30% difference from a general purpose grade 304 to a more corrosion resistant grade 316.

Choosing the right grade of stainless fastener for strength?

As per strength comparison guides, unmarked metric stainless steel fasteners are considered to have a minimum tensile strength of 500 MPa, however most are made to a tensile strength of 700 MPa (as marked A2-70 or A4-70) and some to a tensile strength of 800 MPa (A4-80).

Bumax 109 is a version of 316 stainless steel made to a tensile strength of 1000 MPa (the same as the Super Duplex Bumax SDX109 version) but Bumax Duplex DX129 is made to an incredible tensile strength of 1200 MPa.

Choosing the right grade – based on design and maintenance

Various factors in the design of a fitting or fastener will affect stainless steel grade selection such as surface finish and shape. Performance will also be affected by any cleaning and maintenance schedule for the stainless steel, location, how corrosive the environment is and whether it gets rain washing.