AJKOER
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Education /Awareness Required on Galvanic Corrosion!
Here is a sample on what I am talking about taken from the N2H4 thread:
Quote: Originally posted by franklyn  | From Ye old rhodium apothecary
http://designer-drug.com/pte/12.162.180.114/dcd/chemistry/hy...
Of course you can always just buy the Sulfate, just don't pay more than 15 cents per gram  ( extra with shipping )
http://www.advance-scientific.net/Properties.asp?code=H1019+...
P.S.
@ tito-o-mac
aluminum hydrazine is not a componud , it is a suspension of aluminum dust and anhydrous hydrazine.
Hydrazine hydrate will slowly combine the aluminum with the water into aluminum hydroxide releasing
hydrogen gas until all that remains is anhydrous hydrazine over the aluminum hydroxide precipitate.
Not good if this happens in a closed container having hydrogen under pressure over anhydrous hydrazine.
VERY - B I G - B O O M , house falls back to earth as blizzard of sawdust.
_______________________________________________________________
U P D A T E
Note that my assertion above may be unfounded. Draw your own conclusions.
Quoted from page 392 of Corrosion of Aluminium
posted here _
http://www.sciencemadness.org/talk/viewthread.php?tid=8706&a...
Pure hydrazine and its derivatives monomethylhydrazine (MMH) CH3NHNH2 and
dimethylhydrazine (DMH) (CH3)2NNH2 have no action on aluminium in a wide
range of temperatures between -196 and +27 ºC [14].
Hydrazine solutions have no action on aluminium; the annual decrease in thickness
in solutions at 5 and 10% is below 1 mm.
Hydrazine and its derivatives are not decomposed or altered in contact with aluminium
alloys [15].
This makes it possible to manufacture the tanks of rockets in aluminium alloys
such as 6061. Tests have shown that this alloy is not prone to stress corrosion in
hydrazine [16].
[15] Van der Wall E.M., Suder J.K., Beegle R.L., Cabeal J.A., Propellant/Material compatibility
study, AFRPL, report TR-71-741.
http://stinet.dtic.mil/cgi-bin/GetTRDoc?AD=0736464&Locat...
[16] Gilbreath P., Adamson M.J., The stress-corrosion susceptibility of several alloys in hydrazine
fuels, NASA, report TN D-7604, February 1974.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1974000...
Edited on 25-12-2007
- - - - - - - - - - - - - - - - - - - -
More Curious observations from various sources _
Investigations on the reaction between aluminum and hydrazine have produced
discordant results. However, at least under some conditions, hydrazine has
caused corrosion of aluminum. This corrosion is dependent on the aluminum alloy,
the hydrazine purity and the temperature.
In normal environments aluminum is covered by a protective oxide layer. The
solubility of this oxide in water is very low but is increased in acidic or alkaline
solutions. The oxide is very hard to reduce. For example,the free energy change,
for the reaction 3 N2H4 + 2 Al203 = 3N2 + 6H20 + 4Al, is +321 Kcal. With such a
high positive free energy change, the reduction by hydrazlne is not found.
The solubility of aluminum or aluminum oxide in hydrazine appears to be very slight.
However, when hydrazine was stored in 2014 aluminum, a precipitate was
observed containing aluminum and copper. This dissolution and precipitation
appears to be similar to corrosive reactions in water where there is local solubility
followed by precipitation of the dissolved product.
A commonality exists between solutions of water and carbon dioxide in hydrazine
in that both are sources of protons _
H20 + N2H4 = N2H5(+) + OH(-)
CO2 + N2H4 = H2N2HCOO(-) H(+)
The latter product, carbazic acid, is probably more correctly characterized in
excess hydrazine as the hydrazinium salt, H2N2HCOO(-)N2H5(+). The hydrazinium
ion, N2H5(+), in hydrazine is the analog in properties of the hydronium ion, H30 +,
in water. Simplistically. a measure of the hydrazinium ion concentration may be
calculated for systems containing the two impurities by using the equilibrium
constant for reaction (1), K298 = 8.5X10-7, s and assuming for reaction (2) that
carbazic acid is a strong acid and the CO2 reaction with N2 H4 is complete. Thus,
for small amounts of water, the hydrazinium ion will be proportional to the
half-power of the water concentration and at a 1 percent impurity level would be
7×10 -4 M. Assuming complete ionization, the hydrazinium ion concentration in
reaction (2) is directly proportional to the carbon dioxide impurity
It is expected that the hydrazine salt of carbazic acid will ionize in hydrazine as
N2H5(+) and N2H3C00(-). If the N2H5(+) ion is the reactive or catalytic speciues
in the carbazic acid catalyzed decomposition of hydrazine, this would be in
agreement with the previous observation that, of the other additives tested, only
water increased the decomposition rate.
[Edited on 3-1-2008 by franklyn][/rquote]
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Now Franklyn is IMHO one the best authorities on this forum, but limiting the inputs for the corrosion reaction as "dependent on the aluminum alloy,
the hydrazine purity and the temperature" and pH is potentially, in my opinion, missing a big (think muncipal water tank) one. If one adds the
presence of air, more precisely O2, and the solution having a salt so as to serve as an electrolyte, then one has the makings of an electrochemical
battery cell! The resulting corrosion reaction ( like pitting) will proceed at RT or lower, and will accelerate upon warming and increased air contact
(which happens to be the so called cathodic zone).
Here is an extract from Wikipedia on the Aluminum air battery, link http://en.m.wikipedia.org/wiki/Aluminum_air_battery , to quote:
"Traditional Al–air batteries had a limited shelf life[8] because the aluminium reacted with the electrolyte and produced hydrogen when the battery
was not in use–although this is no longer the case with modern designs. The problem can be avoided by storing the electrolyte in a tank outside the
battery and transferring it to the battery when it is required for use.
These batteries can be used, for example, as reserve batteries in telephone exchanges and as backup power sources. Al–air batteries could be used to
power laptop computers and cell phones and are being developed for such use."
And also:
"Aluminium based batteries
Different types of aluminium batteries have been investigated:
Aluminium-chlorine battery was patented by United States Air Force in the 1970s and designed mostly for military applications. They use aluminium
anodes and chlorine on graphite substrate cathodes. Required elevated temperatures to be operational.
Aluminium-sulfur battery worked on by American researchers with great claims, although it seems that they are still far from mass production. It is
unknown as to whether they are rechargeable.
Al–Fe–O, Al–Cu–O and Al–Fe–OH batteries were proposed by some researchers for military hybrid vehicles. Corresponding practical energy
densities claimed are 455, 440, and 380 Wh/kg[9]
Al-MnO manganese dioxide battery using acidic electrolyte. Produces a high voltage of 1.9 volts. Another variation uses a base (potassium hydroxide)
as the h and sulfuric acid as the catholyte. The two parts being separated by a slightly permeable film to avoid mixing of the electrolyte in both
half cells. This configuration gives a high voltage of 2.6–2.85 volts.
Al-Glass system. As reported in an Italian patent by Baiocchi [[10]], in the interface between common silica glass and aluminium foil (no other
components are required) at a temperature near the melting point of the metal, an electric voltage is generated with an electric current passing
through when the system 6 closed onto a resistive load. The phenomenon was first observed by Baiocchi and after Dell'Era et Al.[11] began the study
and the characterization of this electrochemical system."
As reported in an Italian patent by Baiocchi [[10]], in the interface between common silica glass and aluminium foil (no other components are
required) at a temperature near the melting point of the metal, an electric voltage is generated with an electric current passing through when the
system 6 closed onto a resistive load. The phenomenon was first observed by Baiocchi and after Dell'Era et Al.[11] began the study and the
characterization of this electrochemical system."
The actual half reactions occurring are given by (see https://www.google.com/url?sa=t&source=web&rct=j&... ):
"Anode: Al(s) + 3OH−(aq) → Al(OH)3(s) + 3e−
Cathode: O2(g) + 2H2O(l) + 4e− → 4OH−(aq)
Overall: 4Al(s) + 3O2(g) + 6H2O(l) → 4Al(OH)3(s) "
Now clearly, the reaction at the cathode can raise pH so as to enable the dissolution of the alumina.......
--------------------------------
I created an aqueous version of the battery using Al foil, household dilute ammonia, dilute 3% H2O2 in place of air to speed things up and some sea
salt, a good elecrolyte. What is immediately visible are tiny bubbles (Hydrogen, I suspect). What was unexpected is how in 5 minutes the shiny
Aluminum foil becomes black (Silica or Carbon from the Aluminum alloy used to create the foil?)). White floating particles of Al2O3 are also apparent.
After a day, the reaction cannot be restarted by adding more H2O2 (Al anode is apparently coated as noted in commentary above)......
Now, even better than pure Al are alloys of Aluminum. In general, while alloys can be constructed to be resistant to direct attacks, they often are
better galvanic cells.
I also like cells created from copper, zinc and alloys, and iron (including galvanized steel). See research on copper air, zinc air and iron based
galvanic cells.
This topic is important as lives are lost, and more likely will contine to be lost. As a small example I recall how a worker suffocated upon repairing
an iron pipe that was drained. The rapid start of galvanic corrosion with more surface contact rapidly consumed all the oxygen.
I will let others speculate on why galvanic corrosion continues to be an unexpected in many situations.
[Edited on 5-7-2014 by AJKOER]
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AJKOER
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Here is something that may make you laugh, unless you are a U.S. taxpayer who cares how his tax dollars are used:
| Quote: Originally posted by AJKOER | One thing people should learn is that Aluminum corrosion can be accelerated both by chemical and electrochemical paths. Mixing metals is asking for
galvanic corrosion.
Here is a near billion dollar mistake paid for by the US Navy on its new Aluminum ship (link: http://mobile.bloomberg.com/news/2011-06-17/navy-finds-aggre... ).
The people responsible for this $$$$$ blunder still apparently think it can be fixed! There will have plently of chances to experiment as this ship is
but one of many in a $30 billion fleet.
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blogfast25
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| Quote: Originally posted by AJKOER | "Anode: Al(s) + 3OH−(aq) → Al(OH)3(s) + 3e−
Cathode: O2(g) + 2H2O(l) + 4e− → 4OH−(aq)
Overall: 4Al(s) + 3O2(g) + 6H2O(l) → 4Al(OH)3(s) "
Now clearly, the reaction at the cathode can raise pH so as to enable the dissolution of the alumina.......
--------------------------------
I created an aqueous version of the battery using Al foil, household dilute ammonia, dilute 3% H2O2 in place of air to speed things up and some sea
salt, a good elecrolyte. What is immediately visible are tiny bubbles (Hydrogen, I suspect). What was unexpected is how in 5 minutes the shiny
Aluminum foil becomes black (Silica or Carbon from the Aluminum alloy used to create the foil?)). White floating particles of Al2O3 are also apparent.
After a day, the reaction cannot be restarted by adding more H2O2 (Al anode is apparently coated as noted in commentary above)......
[Edited on 5-7-2014 by AJKOER] |
The reaction at the cathode doesn't raise the pH at all. All hydroxide ions formed by reduction of O2/H2O react with the Al3+ ions, to form Al(OH)3.
The cathode cannot somehow produce an excess of hydroxide ions because that would violate the charge carrier mass balance. The fact that the anode
quickly becomes covered in hydrated alumina poses a serious limitation on these cells. Hence the efforts to 'go chlorine or sulphur'...
As regards the Al foil turning black, I observed the same thing with Magnox fuel cells supplied in a toy kit a few days ago.
http://oxfordchemserve.com/great-toy-great-science-dodgy-eco...
Magnox (Mg, air + salt electrolyte) electrochemistry is very similar to the Al air batteries you talk about.
I'm convinced the blackening is due to etching: small amounts of Al (or Mg) are being removed from the metal surface (and end up as insoluble
hydroxides) thus affecting how the metal reflects light where it is etched. The micro-pitted surface scatters light more thus creating a darker area
where etching has taken place. Other forms of chemical etching produce a similar effect.
[Edited on 5-7-2014 by blogfast25]
[Edited on 5-7-2014 by blogfast25]
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AJKOER
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Blogfast:
Interesting comments.
On violating the charge carrier mass balance, I agree on a total cell basis. However, when one has Al flakes say forming the anodic zone, these are
essentially local electro-chemical half reactions whose products are in close contact with standard chemical competing side reactions, so I am still
somewhat opened minded on a possibly larger array of products.
I seem to be getting better reactions when I employed Al foil that has been heated to red hot (prior thread reference on this suggests gamma weakened
Al2O3 with Al, allowing more rapid removal of the protective Aluminum oxide).
----------------------------------------
I think there may also be an issue of product safety misrepresentation at play as well.
I am getting to the opinion that manufacturer's may be aware of the potential of air and electrolyte exposure causing galvanic corrosion, but are not
able to address it, and just state what their tank can address under test conditions.
Of course, there is always legal language in their operating manual to limit damages. For example, one could insist that storage tanks should be kept
mostly full (which otherwise would allow air to commence a galvanic corrosion attack). Or, if the container previously held a caustic and was not
cleaned, which would also function as an electrolyte, one could require that all tanks be clean following a change in use even though this may be
unlikely in practice.
I believe a better practice is full disclosure and educating the product's user.
[Edited on 5-7-2014 by AJKOER]
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blogfast25
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With Alox cells you're flogging a bit of a dead horse, I think.
I'd experiment with Magnox if I had more time. In particular the influence of catalysts applied to the air cathode on cell characteristics such as
potential and power output.
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