steel in aluminum

[cavalierkid]

Still alittle green behind the ears! What is the chemical reation that happens whin you insert a grade 8 steel bolt in an aluminum block? second what is the difference if you install a zinc plated bolt into that block instead of a regular steel bolt

 

[kenvlach]

None, if you do it right.
The galvanic series for seawater shows the following ranking:
From anodic (active) to cathodic (noble): Zn, Al, steel.
So, it’s a good idea not to submerge your engine block in seawater. For the 1st case (steel in Al), the aluminum will preferentially corrode adjacent to the bolt, possibly seizing the bolt if the liquid reaches the threads. For the 2nd case (galvanized steel in Al), the bolt will seize up due to the formation of voluminous zinc hydroxide if liquid reaches the threads.

For practical advice, use some lubricant or antiseize compound or Loctite on the threads to repel any moisture. If any are ‘wet bolts,’ i.e., they enter a cooling passage, make sure to use the auto manufacturer-recommended sealing compound.

 

[CoryPad]

If you have an uncoated steel bolt contacting an aluminium component, in the presence of an electrolyte such as salt spray, then you can expect a chemical reaction between aluminium and oxygen to form aluminium oxide. This results in dissolution of the structural aluminium part, so this is undesireable.

If you have a zinc coated steel bolt contacting an aluminium component, in the presence of an electrolyte, there may or may not be a reaction. The zinc coatings are complex (they are passivated with hexavalent or trivalent chromium), so predicting the results is complex.

The zinc plating is used to prevent the steel bolt from corroding (chemical reaction between iron and oxygen to form iron oxide). This reaction also creates hydrogen, which can enter the steel and cause delayed cracking, which is undesireable. Also, automobile customers find visible iron oxide to be displeasing, so automobile manufacturers coat the bolts to prevent corrosion.

Current automotive practice regarding steel bolts and aluminium parts is to use an aluminium containing coating for the bolt, which prevents significant corrosion of the aluminium parts.