corrosive spray - H2 generation ? 2

[jdkuhndog]

Greetings !

I am reading out of ASME NOG-1-2002, "Rules for construction of overhead and gantry cranes".

Paragraph 1145 Chemical (a) Spray Systems

"...Any restriction on the use of materials due to the effects of the spray shall also be specified. Specifically, where a corrosive spray is present the possibility of H2 generation exists and, therefore, the use of exposed aluminum, magnesium, galvanized steel, and zinc is to be minimized."

What does this mean ? how is H2 generated ? is it generated because corrosive spray comes in contact with the listed materials ? or since it is somehow generated, the listed materials should not be used ?

Forgive my ignorance on this subject.

Thanks !

 

[Metalguy]

The H2 is generated during corrosion. It actually is formed as atomic H at the cathodic sites on the metal, where it helps reduce/stop the corrosion process (polarization). But H has a very strong tendency to become H2.

Incidently, the presence of oxygen removes H from the cathodes (depol.), which is one reason why oxygen increases the corr. rate.

 

[jdkuhndog]

Thanks Metalguy.

follow up question...


"...the possibility of H2 generation exists and, therefore, the use of exposed aluminum, magnesium, galvanized steel, and zinc is to be minimized."

Why should the use of the listed materials be minimized since H2 exists ? It's not like they are going to explode if they come into contact with H2 ! Wouldn't you want to use galvanized steel where there is a 'corrosive spray' ?

 

[kenvlach]

Two very good reasons:
1) These elements are sufficiently reactive that a significant quantity of explosive H₂ gas could rapidly form, perhaps rising into and accumulating in an electrical enclosure.
Remember the high school chemistry demosnstration,
Zn + 2 HCl(aq) --> ZnCl₂(aq) + H₂(g)

2) If these materials are used as a coating or otherwise in contact with high strength steel, the H atoms released from the reaction could diffuse into the steel, thereby causing hydrogen embrittlement.

 

[jdkuhndog]

Thanks kenvlach - that helps !