AJKOER
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Salt water is not best described as corrosive with anodic metals
Perhaps more like a good electrolyte, to quote a source:
"Yes, it accelerates it . Water is the enabler of fast oxidation of iron so freshwater will also cause rust. However, salt water is a very good
conductor (lots of dissociated ions) and so there are a number of electrolysis reactions that tremendously accelerate corrosion in salt water.”
Source link: http://scienceline.ucsb.edu/getkey.php?key=552
My take on the reaction of metals like, Iron, Aluminum, Magnesium, for example, with water in the presence of a good electrolyte like NaCl:
H2O = H+ + OH-
Fe + 2 OH- → Fe(OH)2 + e-
H+ + e- = .H
.H +.H → H2
…….
Now, in the case of Aluminum, indeed more possible electrochemical reactions, which I would express as follows, in the presence of an electrolyte like
NaCl:
H2O = H+ + OH-
Al + 3 OH- → Al(OH)3 + 3 e-
H+ + e- = .H
.H +.H → H2
.H + e- = H-
Al (surface)→ Al(3+) + 3 e-
Al(3+) + 3 H- →AlH3
AlH3 + 3 H2O → Al(OH)3 + 3 H2
……
To quote a source:
“”In addition to being a primary corrosion process, dissolution behavior of aluminum and its alloys in alkaline solutions is of considerable
interest because it is the anode reaction in aluminum-air batteries.[4] ......The anodic half-reaction at the Al electrode is
Al + 4 OH − → Al(OH)4− + 3 e− (1.1)
which exhibits an electrode potential of -2.35 V in alkaline solutions(vs. NHE).
"2 Al + 6 H2O → 2 Al(OH)3 + 3 H2 (1.2)"
Also:
....."dissolution of aluminum in alkaline solutions at open-circuit also leads extremely high rates of H-absorption into the metal, [9-14] ".....
"Another study of the dissolution of aluminum in aqueous solutions by Perrault revealed that the open circuit potential of aluminum in strongly
alkaline solutions corresponds closely to the Nernst potential for oxidation of aluminum hydride to aluminate ions [25]
AlH3 + 7 OH− (aq) → Al(OH)4− + 3 H2O ( aq ) + 6 e− (1.3)
This suggests a role of surface aluminum hydride as a reaction intermediate in the dissolution process. Additional evidence for the presence of
aluminum hydride was provided by Despic and co-workers.[26, 27] They found that aluminum hydride formation was one of the major processes apart from
aluminum dissolution and hydrogen evolution, during the cathodic polarization of aluminum. Titanium corrosion in alkaline solutions is also thought to
proceed through a hydride mediated mechanism.[28-30] "
"He found that the open-circuit potential in strongly alkaline media was determined by the equilibrium of the reaction
AlH3 + 7 OH− (aq) → Al(OH)4- + 3 H2O ( aq ) + 6 e− (3.7)
The obtained a standard chemical potential of 25 kcal/mol for AlH3 from his data, which was in reasonable agreement with prior thermochemical
calculations done by Sinke et al who obtained a value of 11.1 kcal/mol for the chemical potential.[80] ...."
Source: A 2008 doctoral thesis , "Alkaline dissolution of aluminum: surface chemistry and subsurface interfacial phenomena", by Saikat Adhikari, link:
http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=16827&...
[Edited on 30-10-2017 by AJKOER]
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Sulaiman
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I mostly get what you wrote, but there is no disproof of salt water being corrosive.
e.g. accepting that salt acts as an ionic transport mechanism increasing electrolyte conductivity,
are we sure that the Na+ and/or Cl- ions have no effect at the electrode/electrolyte boundary ?
CAUTION : Hobby Chemist, not Professional or even Amateur
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AJKOER
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Sulaiman:
On your question: "are we sure that the Na+ and/or Cl- ions have no effect at the electrode/electrolyte boundary?" here is an article that may address
your point, but a bit advanced in the realm of physical chemistry, "Adsorption of Singly Charged Ions at the Hydroxylated (0001) α-Quartz/Water
Interface" by Morgane Pfeiffer-Laplaud and Marie-Pierre Gaigeot, in J. Phys. Chem. C, 2016, 120 (9), pp 4866–4880, DOI: 10.1021/acs.jpcc.5b10947 .
To quote from the abstract:
"Individual alkali (Na+, K+) and halide (Cl–, I–) ion effects have been characterized at the fully hydroxylated (0001) α-quartz water interface
by means of ab initio molecular dynamics simulations in the framework of the electronic DFT representation (DFT-MD). We particularly focus our
analyses on the ion adsorption and solvation structures (made by water and by surface silanols), as well as on perturbations undergone by the silanol
surface sites when comparing the charged interfaces (present work) to the neat interface (our previous works, J. Chem. Theory. Comput. 2012, 8, 1037;
J. Phys.: Condens. Matter 2012, 24, 124106). Both sodium and potassium cations are found adsorbed in an inner-sphere configuration, while chloride and
iodide are found in between inner- and outer-sphere. Cation adsorption at the interface is found to induce more perturbation on interfacial properties
than anions do. In particular, we show in details how and why the orientation of out-of-plane and in-plane surface silanols found at the neat
interface are modified by inner-sphere cations at the charged interfaces, with also consequences on the silanol–silanol intrasurface hydrogen bond
network. All this detailed analysis provides a clear picture of a reduction of acidity of the surface silanols at the quartz/water interface in the
presence of the alkali/halide salts."
Link: http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.5b10947?journal...
The possible reduction in surface acidity could promote reactions (1.3) and (3.7) as I noted previously above.
[Edited on 1-11-2017 by AJKOER]
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AJKOER
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In the case of a ferrous/O2/H+ reaction system, metal autooxidation (see http://pubs.acs.org/doi/abs/10.1021/ja01600a004 ) is believed to be accelerated in the presence of cupric, likely found in sea water. The claimed
redox couple equilibrium reaction, in which cuprous is created and promotes a redox reaction, is:
Fe(ll) + Cu(ll) = Fe(lll) + Cu(l)
The issue with the above is that cuprous is not usually soluble. However, in the presence of NaCl, a soluble salt can be formed:
NaCl (aq) + CuCl = Na[CuCl2] or NaCl-CuCl-H20 (a polynuclear complex, see http://iopscience.iop.org/article/10.1070/RC2000v069n11ABEH0... )
This is one step in a commercial preparation path to basic cupric chloride (see Equation 6 at https://en.wikipedia.org/wiki/Dicopper_chloride_trihydroxide ).
So NaCl (or KCl) could contribute to the solubility of cuprous, which supports the Fe/Cu redox couple equilibrium that promotes a Fe(ll)/O2 redox
reaction.
[Edited on 1-11-2017 by AJKOER]
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unionised
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Get two pairs of cheap stainless steel scissors.
soak one in salt water and the other in fresh water.
Don't dry them, just lave them for a week or so.
See if you still think salt water isn't corrosive.
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SWIM
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Was that a typo, or a bit of indulging in archaic English usage?
Makes perfect sense either way.
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AJKOER
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Sea water has a pH of about 8.1, which is not very corrosive in itself.
Your two pairs of cheap stainless steel scissor if left in contact with air can still both still undergo some electrochemical reactions resulting in
corrosion with time, especially with salt water. See my thread with pictures on what happens with iron filings with some sea salt in air at http://www.sciencemadness.org/talk/viewthread.php?tid=77204 .
But, what is promoting the corrosion reaction, that pH of 8.1 or is it other electrochemical and/or redox reactions accelerated in the presence of sea
salt?
So far, this thread addresses the role of OH- in electrochemical reactions, promoted by the good electrolyte consisting of aqueous sea salt, and
interestingly, even a possible complexing between NaCl and cuprous, to further advance other possible redox reactions involving low a valent state
transition metal and oxygen.
Bottom line, sea water is itself not too corrosive at a pH of 8.1, but via its property of being a good electrolyte and, a possible complexing agent,
may foster (or be an 'enabler' of) reactions inducing metal corrosion. The particular metals in question have a high anodic index (see http://www.zygology.com/cms/upload_area/pdf/Zyg-Anodic-Index... ).
[Edited on 2-11-2017 by AJKOER]
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Sulaiman
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AJKOER, there is a semantics problem here - please give your definition 'corrosive'
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AJKOER
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OK, per Wikipedia (see https://en.wikipedia.org/wiki/Corrosive_substance ), to quote:
"Chemical terms
The word corrosive is derived from the Latin verb corrodere, which means to gnaw, indicating how these substances seem to "gnaw" their way through
flesh or other material. Sometimes the word caustic is used as a synonym but caustic generally refers only to strong bases, particularly alkalis, and
not to acids, oxidizers, or other non-alkaline corrosives."
Per the definition above, sea water is not caustic, a possible synonym of corrosive, as it is not a strong base (pH 8.1).
Hence my (and possible others per my opening reference) that sea water is better described as an enabler of electrochemical electrode destruction
being an electrolyte. Note, electrochemical reactions can involve an induction period which is not usually associated with caustic/corrosive
compounds.
[Edited on 2-11-2017 by AJKOER]
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