Tuesday, August 14, 2012

Galvanic Corrosion & BUMAX fasterners

One of the biggest questions I get today is about Galvanic corrosion and can BUMAX help.  The answer is yes, but it is not the end all product.  While the inherent properties of BUMAX reduces galvanic corrosion, it doesn’t fully prevent it.  BUMAX brand can be coated to provide maximum protection while retaining all the properties of 316L but with the increased strength.  BUMAX Fasteners for Galvanic Corrosion due exist.  If you would like more information email chris.portell@bufab.com  or look at www.BUMAX.us .  Below is additional information on what is Galvanic Corrosion.

 Galvanic corrosion, also called Bimetallic Corrosion, takes place when two dissimilar metals are electrically connected in an electrically conducting fluid.

The likelihood and severity of galvanic corrosion depends on several factors like:

  1. Corrosion potential of metals in the environment considered
  2. Relative area of the two metals
  3. Type of electrolyte

The corrosion potentials of the stainless steelsare noble than the corrosion potentials of aluminum, as can be seen in the galvanic series of metals in seawater below.

This mean that there will be no galvaniccorrosion on stainless steel when placed in contact with aluminum while aluminum will corrodes.

Permissible couples to avoid galvanic corrosion of aluminium when placed in contact with more noble materials are limited to a potential difference of 0.10 V on the galvanic series.

Anodic current density and hence corrosion rates can be reduced by increasing the anode-to-cathode surface area ratio; if a small area of stainless steel is placed in contact with a big area of aluminum the rate of aluminum galvanic corrosion is low due to the effect of the relative areas.

It is possible to avoid stainless steel aluminium galvanic corrosion isolating the two material by means of an electrical insulating material, like rubber.

In the presence of crevices stainless steels may exhibit less noble potentials due to oxygen depletion within the crevice.

Therefore, coupling a relatively large area with the small-area characteristic of a crevice may result in rapid attack of the material within the crevice leading to stainless steel corrosion.

Under some circumstances, coupling stainless steel to a more active metal can shift the stainless steel corrosion potential from a passive range to an active range. This shift results in an increase in the corrosion rate of the stainless steel due to stainless steel galvanic corrosion.

Stainless steel fasteners in contact with more active metals can become embrittled from the hydrogen
generated by the stainless steel aluminum galvanic corrosion couple.

The table below reports the Corrosion potentials in flowing sea water at ambient temperature.

The unshaded symbols show ranges exhibited by stainless steels in acidic water such as may exist in crevices or in stagnant or low velocity or poorly aerated water where Stainless Steel become active, while the shaded areas show the potentials of Stainless Steel when is in passive state.

This table is taken from: Atlas Steel Technical Note No. 7 "Galvanic Corrosion"

1 comment:

  1. Galvanic Corrosion can bs such a problem.  Even for an excellent Stainless Steel Alloy such as Bumax Stainless - Seperating the Aluminium and Stainless is definately the best solution, an isolating gel such Tef Gel seems to work well.