AJKOER
Radically Dubious
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Registered: 7-5-2011
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A New Hypothetical Advanced Look at the Rusting of Iron
Isn't chemistry wonderful, we're in the year 2020, and I'm still arguing about the likely mechanics on the rusting of iron!
I start with a simple background presentation of rust formation per a typical source: https://www.wonderopolis.org/wonder/why-do-some-things-rust, to quote:
“Rust occurs when iron or its alloys, such as steel, corrode. The surface of a piece of iron will corrode first in the presence of oxygen and
water...Other substances, such as salt, can also increase the speed of the rusting process.”
Also, continuing:
“Rust forms when iron and oxygen react in the presence of water or moisture in the air.”
More accurate, extensive, and more complex is Wikipedia on rust (see https://en.wikipedia.org/wiki/Rust), to quote:
“Rust is an iron oxide, a usually reddish brown oxide formed by the reaction of iron and oxygen in the presence of water or air moisture. Several
forms of rust are distinguishable both visually and by spectroscopy, and form under different circumstances.[1] Rust consists of hydrated iron(III)
oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3)."
And further of particular interest on the mechanic of rust formation:
“The hydrogen atoms present in water molecules can combine with other elements to form acids, which will eventually cause more metal to be exposed.
If chloride ions are present, as is the case with saltwater, the corrosion is likely to occur more quickly….the corrosion of most metals by oxygen
is accelerated at low pH. Providing the electrons for the above reaction is the oxidation of iron that may be described as follows:
Fe → Fe(2+) + 2 e−
The following redox reaction also occurs in the presence of water and is crucial to the formation of rust:
4 Fe(2+) + O2 → 4 Fe(3+) + 2 O(2−)“
The last above reaction is questionable, in my opinion (at least where the surface pH<4.88), as the major REDOX reaction (as will be detailed
below). Wikipedia then cites several possible hydrolysis reactions as the likely needed source for H+, to quote again from Wikipedia:
“In addition, the following multistep acid–base reactions affect the course of rust formation:
Fe(2+) + 2 H2O ⇌ Fe(OH)2 + 2 H+
Fe(3+) + 3 H2O ⇌ Fe(OH)3 + 3 H+
as do the following dehydration equilibria:
Fe(OH)2 ⇌ FeO + H2O
Fe(OH)3 ⇌ FeO(OH) + H2O
2 FeO(OH) ⇌ Fe2O3 + H2O
From the above equations, it is also seen that the corrosion products are dictated by the availability of water and oxygen. “
Now, I completely agree with this last sentence. So, what is my plausible challenge to what has been primarily an electrochemical and standard
chemistry path?
My suggested rendition involves commonly occurring transition metal radical chemistry (inclusive of iron and possible alloys present) along with the
likely presence of a surface chemistry aspect. More precisely, the so-called metal auto-oxidation reaction citing the creation, in a reversible
reaction, of the superoxide radical anion per the reactions:
Fe + 2 O2 <-> Fe(II) + 2 •O2(−)
Fe(II) + O2 <-> Fe(III) + •O2(−)
Further, on the surface of iron (or the surface of the ferrous solution), the superoxide radical anion exists as the hydroperoxyl radical (•HO2, pKa
4.88), as the superoxide radical anion will extract an H+ even from water vapor when the superoxide is in a medium of air. Of import, this article
on acidic radicals: ‘Radical-Enhanced Acidity: Why Bicarbonate, Carboxyl, Hydroperoxyl, and Related Radicals Are So Acidic’ at https://pubmed.ncbi.nlm.nih.gov/28915030/ . It is my apparently new claim that the acidic Hydroperoxyl radical could be the major contributor to
the required presence of H+.
Further, I cite an electrochemical reaction occurring with iron (and also other transition metals) in the presence of air/oxygen and H+:
4 Cu/ 2 Fe/ 2 Co/ 2 Cr... (aq) + O2 + 2 H+ --> 4 Cu(l)/ 2 Fe(ll)/ 2 Co(ll)/ 2 Cr(ll) + 2 OH-
Or, as more frequently cited in practice:
4 Cu(l)/Fe(ll)/Co(ll)... (aq) + O2 + 2 H+ --> 4 Cu(ll)/Fe(lll)/Co(lll) + 2 OH-
The above reaction can also be derived citing a series of radical chemistry reactions. Namely,
per a 2013 radical reaction supplement, "Impacts of aerosols on the chemistry of atmospheric trace gases: a case study of peroxides
radicals"', by H. Liang1, Z. M. Chen1, D. Huang1, Y. Zhao1 and Z. Y. Li, link: https://www.google.com/url?sa=t&source=web&rct=j&... , I note the following reactions:
R24 O2(aq) + Cu+ → Cu2+ + O2− ( k = 4.6xE05 )
R27 O2− + Cu+ + 2 H+ → Cu2+ + H2O2 ( k = 9.4xE09 )
R25 H2O2 + Cu+ → Cu2+ + •OH + OH− ( k= 7.0 xE03 )
R23 •OH + Cu+ → Cu2+ + OH− ( k = 3.0×E09 )
Where the implied net reaction, in the case of copper, confirms the claim reaction above:
Net reaction: O2 + 4 Cu+ + 2 H+ → 4 Cu2+ + 2 OH-
Interestingly, there is also an electrolysis based reference: See p. 7 at https://www.utc.edu/faculty/tom-rybolt/pdfs/electrochem2014.... for the reverse reaction with 2 H+ adding to each side. Alternate source of the
above reaction, per my records, but access to the full article is no longer free, see: https://www.researchgate.net/publication/262451840_Review_of... .
Also, apparently, the cited net reaction is actually part of the underlying chemistry for the commercial preparation of, for example, basic copper
chloride (see Eq 7 at https://en.wikipedia.org/wiki/Dicopper_chloride_trihydroxide ).
So, to summarized, the key difference between the Wikipedia version on the mechanics on iron rust formation, is that my rendition centers on the
employment of a well-known redox reaction occurring with various transition metals (like metal auto-oxidation creation of the superoxide radical) and
the recognition of possible surface chemistry (where pH <4.88), thereby leading to the associated somewhat unique acidic properties of the
hydroperoxyl radical. The latter could hypothetically account for the presence of the required H+ to source an electrochemical oxidation reaction
(also cited for many transition metals) resulting in the attack of the metal.
[Edited on 4-8-2020 by AJKOER]
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AJKOER
Radically Dubious
Posts: 3026
Registered: 7-5-2011
Member Is Offline
Mood: No Mood
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Found an additional advanced confirming source ‘ Thermodynamics of Oxidation of Iron and Carbon Steels in Water (available at https://link.springer.com/content/pdf/bbm%3A978-90-481-3477-... ), to quote:
“A.2 Thermodynamics of Oxidation of Ferrous Ions with Oxygen
Fe2+ (aq) + O2(aq) + H+ (aq) → Fe3+ (aq) + HO2(aq), G◦298 = +86.95kJ/mol, (A.9)
2 Fe2+ (aq) + O2(aq) + 2 H+ (aq) → 2 Fe3+ (aq) + H2O2(aq), G◦298 = +12.8kJ/mol,(A,10)
4 Fe2+ (aq) + O2(aq) + 4 H+ (aq) → 4 Fe3+ (aq) + 2H2O(l), G◦298 = −177.40 kJ/mol. “
The first equation supports my formation of the •HO2 species (I enumerated the step, namely direct interaction with oxygen) and the last equation is
yet another reference to my cited key electrochemical oxidation net reaction for ferrous (upon adding 2H+ to each side of the equation for neutral
conditions). In fact, the author also provides a version for basic conditions:
2 Fe2+ (aq) + 2OH− + O2(aq) + 2H2O(l) → 2Fe(OH)3(s), (C.4)
The author also cites the existence of surface absorbed hydrogen species per the equation:
H(ads) + H(ads) → H2(g). (C.5)
And in words:
“H(ads) presents the adsorbed hydrogen atom on the metal surface.”
More cited reactions of interest include:
3Fe(OH)2(s) → Fe3O4(s) + 2H2O(l) + H2(g), (A.5) (per again https://link.springer.com/content/pdf/bbm%3A978-90-481-3477-... )
2Fe(OH)3(s) → Fe2O3(s) + 3H2O(l), (A.6) (same source)
Fe(s) + 2Fe3+ (aq) → 3Fe2+ (aq), G◦298 = −233.5 kJ/mol, (A.12) (same source)
Fe(s) + 6H2O(l) → [Fe(H2O)6]2+ (aq) + 2e−, (B.12) (same source)
Found two more supporting sources, per the first (at https://faculty.kfupm.edu.sa/ME/hussaini/Corrosion%20Enginee... ), to quote relevant half-cell reactions:
“(1) O2 + 4H+ + 4e- -> 2H2O (if the water is acidic)
(2) O2 + 2H2O + 4e- -> 4H+ (if the water is basic)
The negatively charged OH- ions react with the positively charged Fe2+ ions and forms Fe(OH)2 .”
The second source (available at https://pubmed.ncbi.nlm.nih.gov/17368726/ ) also relates to pH and also to iron speciation, to quote from the article: 'The effect of pH on the
kinetics of spontaneous Fe(II) oxidation by O2 in aqueous solution--basic principles and a simple heuristic description’, to quote:
“This rate equation yields a sigmoid-shaped curve as a function of pH; at pH values below approximately 4, the Fe(2+) concentration dominates and
the rate is independent of pH. At pH> approximately 5, [Fe(OH)(2)(0)] determines the rate because it is far more readily oxidized than both Fe(2+)
and FeOH(+). Between pH 5 and 8 the Fe(OH)(2)(0) concentration rises steeply with pH and the overall oxidation rate increases accordingly. At pH
values> approximately 8 [Fe(OH)(2)(0)] no longer varies with pH and the oxidation rate is again independent of pH.”
The above implies both a pH consideration relating to iron speciation, which correspondingly likely specifies the appropriate reaction equation.
Also per Science Direct (at https://www.sciencedirect.com/topics/engineering/corrosion-m...), to quote an important point I cited, different chemistry occurs near/at the
surface:
“On a metal surface exposed to atmosphere, only a limited quantity of water and dissolved ions are present, whereas the access to oxygen present in
the air is unlimited [1]. Corrosion products are formed close to the metal surface, unlike the case in aqueous corrosion,...”
[Edited on 7-8-2020 by AJKOER]
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