Corrosion and galvanic corrosion (Basic level)

[Juanda]

TL;DR Summary

I'm trying to understand galvanic corrosion, the rate at which it'd happen given some conditions, and some practical means to reduce its impact. For that, I first need a deeper understanding of normal corrosion too.

Let's start with something simple. Imagine we have a block of pure iron on an electrically insulated table (maybe plastic). It's exposed to the normal atmosphere you'd find in a city several kilometers away from the sea.
According to this link, this would be the reaction.

Rust is a combination of several different oxides of iron. The equations below show the steps involved in one of the many processes of rust formation.
$$\begin{align*} 2 \ce{Fe} \left( s \right) + \ce{O_2} \left( g \right) + 4 \ce{H^+} \left( aq \right) &\rightarrow 2 \ce{Fe^{2+}} \left( aq \right) + 2 \ce{H_2O} \left( l \right) \\ 4 \ce{Fe^{2+}} \left( aq \right) + \ce{O_2} \left( g \right) + 6 \ce{H_2O} \left( l \right) &\rightarrow 2 \ce{Fe_2O_3} \cdot \ce{H_2O} \left( s \right) + 8 \ce{H^+} \left( aq \right) \end{align*}\nonumber$$
Iron is first oxidized to iron (II) ions by oxygen. In the second step, the iron (II) ions are further oxidized and combine with water and oxygen gas to produce a hydrated form of iron (III) oxide known as rust. Rusting is one of many examples of corrosion. Corrosion is the deterioration of metals by redox processes. Corrosion causes enormous amounts of damage to buildings, bridges, ships, cars, and other objects. It has been estimated that corrosion costs the U.S. economy over $100 billion each year. A great amount of time and effort is spent to try to limit or prevent corrosion.

I have some questions about that chemical formula regarding "normal" corrosion.

1. Are there hydrogen ions floating in an aqueous solution in the atmosphere? It seems it's a requirement for the oxidation process to happen but I find that strange since I don't know how it can be in an aqueous solution in gas and because ions tend to neutralize at the first given chance because the electric forces are very strong (compared to gravity for example) so I don't see how it can last long enough hanging there until the chance to react comes its way.
2. How would the proximity to the coast magnify or accelerate the chemical reaction? I assume it must be related to the greater humidity and the possible presence of $NaCl$ which might combine with $H$ or something like that but I can't find the chemical reaction put in terms like what's shown above.

Then, for galvanic corrosion.

Imagine now there is a block of copper on top of the block of iron. First, visualize it in the same scenario far away from the coast. We have two dissimilar metals in contact and the atmosphere will be the electrolyte.
Can you provide what would be the chemical reaction in such a scenario? When I checked iron-copper galvanic cells, it seemed the conditions were more strict than just standing there in electrical contact and exposed to the atmosphere. I mean, if there is no copper available in the atmosphere, how can the galvanic reaction occur?

Lastly, how would the process be accelerated if the experiment were repeated near the coast?

Later I'd like to propose a few more scenarios with different materials but I think this is a proper starting point to get a grasp on the topic.

Thanks in advance

 

[.Scott]

If the two metals are in contact only with each other and the air, there is very little possibility of an electric current, so there would be little effect on corrosion.

But the situation is much different with sea water:
Marine Corrosion Protection
Zinc if often used as a "sacrificial anode".

 

[Lnewqban]

Normal atmospheric conditions keep some water vapor dissolved in the air.
Galvanic corrosion in tropical regions happens in “dry conditions” as well.
Temperature is another factor to consider, as more energetic molecules work harder on interaction.

 

[Juanda]

This thread is mainly motivated by the following documents.

1. ECSS-Q-ST-70-14C (1 November 2016): Corrosion
2. ECSS-Q-ST-70-36C (6 March 2009): Material selection for controlling stress-corrosion cracking
3. ECSS-E-HB-32-23A Rev.1 (6 February 2023): Threaded fasteners handbook (I could only find the online version of the 2010 release)
4. ECSS‐Q‐ST‐70C6 (March 2009): Materials, mechanical parts and processes

So galvanic corrosion is a big issue as you can see in the picture. In working conditions, it has no effect because in space there is no electrolyte. However, it is necessary to ensure that degradation during manufacturing and storage until launch is controlled.

Ideally, all products will always be in a class 6 corrosive environment. However, ideal scenarios and reality tend to differ so I'm trying to get a firmer grasp on corrosion.
That is the origin of my interest in knowing the exact chemical reactions happening during corrosion.

Some other points I'm trying to understand in more detail are:

• Galvanic compatibility is not always respected. According to Table 5‐1 Compatible couples for bimetallic contacts from ECSS‐Q‐ST‐70C (2009), aluminum should not be used in contact with copper without taking special measures. However, it's a common practice to install locking bronze helicoils to avoid galling. It's even recommended in ECSS-E-HB-32-23A Rev.1 (6 February 2016). Even the use of stainless steel fasteners in contact with aluminum is cause for concern. I assume in those cases it's often allowed because the aluminum chunk (the least noble of the couple so the one that will corrode) is often significantly bigger than the helicoil or fasteners so, since the exposed area ratio between the anode and cathode is so big, it's assumed that the corrosion will happen at a very slow rate.
  
  
  
  
  
  
  
  
  By the way, I find it very annoying the aluminum alloys are not classified as 1000, 2000, etc in Table 5.1. I checked the chemical composition so I believe I know which is which but I'll always live with that doubt.
• Prevention of bimetallic corrosion: One of the alternatives to avoid this corrosion is to avoid electrical contact. I have learned that, in case of opting for this route, it's always preferable to protect the cathode. That way, if the coating is damaged (maybe scratched while assembling the part with screws), the area ratio will still be favorable. However, in ECSS-E-HB-32-23A Rev.1 (6 February 2023) I found the opposite recommendation which makes no sense to me because if the coating fails at a point, corrosion will be focused on that point having a greater impact. An example would be anodizing an aluminum component that will be threaded into place with stainless steel screws.
  
  
  
  
  
  
  I must be misunderstanding something about this point because ECSS documentation is written by people way more experienced and qualified than me but I don't see how to marry what I knew already about the prevention of galvanic corrosion and this piece of information.

 

[Nik_2213]

IIRC,, unless the substrate is monolithic, lacking crystal boundaries, and is perfectly clean, you will get local electrochemical differences.

If the environment can provide an electrolyte, game on !!

There's a tangential 'gotcha', whisker formation. A known failure mode of certain types of eg semiconductor on-chip connections.
Cost us some very nice digital balances which became unreliable due to 'hairline' growths in force-bridge gap. Scant effect on 'gross' loads,, such as the internal calibration procedure, but could stall if you were adding powder literally fleck by fleck. As I understand it, traces of solvent in lab air locally pin-holed the protective varnish on the force-bridge...

 

[Juanda]

IIRC,, unless the substrate is monolithic, lacking crystal boundaries, and is perfectly clean, you will get local electrochemical differences.

If the environment can provide an electrolyte, game on !!

What do you mean by substrate?
Also, I would like to know in which scenarios the environment can provide an electrolyte. That was one of the main points of the original OP. Is there documentation about that?

There's a tangential 'gotcha', whisker formation. A known failure mode of certain types of eg semiconductor on-chip connections.
Cost us some very nice digital balances which became unreliable due to 'hairline' growths in force-bridge gap. Scant effect on 'gross' loads,, such as the internal calibration procedure, but could stall if you were adding powder literally fleck by fleck. As I understand it, traces of solvent in lab air locally pin-holed the protective varnish on the force-bridge...

Whysker formation is a known issue in the space industry. It's addressed by different norms such as ECSS-Q-70-71A rev. 1 (18 June 2004) and NASA documentation. Shouldn't be a problem as long as those materials are not used, right?

 

[Astronuc]

TL;DR Summary: I'm trying to understand galvanic corrosion, the rate at which it'd happen given some conditions, and some practical means to reduce its impact. For that, I first need a deeper understanding of normal corrosion too.

There will always be an electrochemical potential between different elements. That is the basis of batteries, and corrosion.

One needs a conductor between the two electrodes (anode and cathode). In the environment, water/moisture serves as the conductor.

Separately, low melting point metals are problematic because they can migrate under the right thermal conditions, which is why there are restrictions on their use in vacuo, or certain electronics, especially power electronics which can operation under 'hot' conditions.

Cadmium	594.18 K	321.18 °C
Lead	600.6 K	327.6 °C
Zinc	692.73 K	419.73 °C

Ref: https://angstromsciences.com/melting-points-of-elements-reference

One way to prevent corrosion is to passivate a metal by forming a 'protective' oxide on the surface. If however, the oxide cracks, then one can get a localized corrosion where the underlying metal (substrate) is exposed to a concentrated electrolyte. Pitting or grain boundary attack is a concern.

Basically, metals tend to want to go back to a lower energy state, which means combining with available oxygen to form oxides - the natural state of metal ores.

 

[Astronuc]

TL;DR Summary: I'm trying to understand galvanic corrosion, the rate at which it'd happen given some conditions, and some practical means to reduce its impact. For that, I first need a deeper understanding of normal corrosion too.

Are there hydrogen ions floating in an aqueous solution in the atmosphere? It seems it's a requirement for the oxidation process to happen but I find that strange since I don't know how it can be in an aqueous solution in gas and because ions tend to neutralize at the first given chance because the electric forces are very strong (compared to gravity for example) so I don't see how it can last long enough hanging there until the chance to react comes its way.

There are hydrogen ions and hydroxyl ions in equilibrium in water, and there is dissolved oxygen in the water, which is how fish 'breathe' underwater. There is also CO₂ dissolved in water, which tends to make the water acidic through the formation of carbonic acid.

https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-acidification
https://newscenter.lbl.gov/2014/10/22/new-insights-on-carbonic-acid-in-water/

There is also moisture in the air. And let us not forget sunlight, especially UV, which can provide energy to drive chemical reactions. Note the phytoplankton and underwater plants absorb light and CO₂ and produce O₂ through photosynthesis.

https://earthobservatory.nasa.gov/features/Phytoplankton
https://www.nature.com/articles/483S17a
https://pubmed.ncbi.nlm.nih.gov/24311124/
https://scripps.ucsd.edu/news/pheno...ver-cellular-process-behind-oxygen-production

In water, one has to be concerned with biofilms on metal surfaces. Note that in seawater, ships are usually painted, if the hulls and structures are carbon or low alloy (ferritic) steels. Some ships are made of Al, while some ships may be constructed of special stainless steels, e.g., AL-6XN, and some structures might be made of 254SMO, or 654SMO, so-called high Mo stainless steels, or more commonly superaustenitic stainless steels.

For example, https://onepetro.org/NACECORR/proceedings-abstract/CORR96/All-CORR96/NACE-96508/113993.
Article mentions an alloy, 25 Cr-22Ni-5.8Mo-1.5Cu-2W-O.45N, which appears to be alloy UNS S39274, or a derivative, which is apparently a super-duplex stainless steel (SDSS). I have to do more research on this alloy.

NACE (National Association of Corrosion Engineers), which became NACE International, recently became AMPP (Association for Materials Protection and Performance), when NACE International merged with the Society for Protective Coatings (SSPC) in 2020/2021.

 

[Astronuc]

TL;DR Summary: I'm trying to understand galvanic corrosion, the rate at which it'd happen given some conditions, and some practical means to reduce its impact. For that, I first need a deeper understanding of normal corrosion too.

How would the proximity to the coast magnify or accelerate the chemical reaction? I assume it must be related to the greater humidity and the possible presence of which might combine with

1. or something like that but I can't find the chemical reaction put in terms like what's shown above.

Then, for galvanic corrosion.

I should point out that the hollow or open boxes are the passivated forms of the metal/alloy, in which a protective oxide has formed on the surface of the metal/alloy.

Marine and coastal corrosion is complicated because the electrolyte (seawater, brackish water, rainwater, moisture) changes with seasons and weather. The main concern with seawater, and sea spray, is the halide salts, primary NaCl and MgCl₂, the latter of which is very corrosive (see Mg in the galvanic series). The chlorine atom attacks the oxide layer and can break it down such that the protective oxide layer is no longer protective. Near some coast areas, primarily oil refineries and urban areas, is the presence of sulfur compounds, due to sulfur in petroleum, and sulfates and polythionic compounds are also detrimental to corrosion resistance, espcially in conjunction with Cl compounds.

Imagine now there is a block of copper on top of the block of iron. First, visualize it in the same scenario far away from the coast. We have two dissimilar metals in contact and the atmosphere will be the electrolyte.
Can you provide what would be the chemical reaction in such a scenario? When I checked iron-copper galvanic cells, it seemed the conditions were more strict than just standing there in electrical contact and exposed to the atmosphere. I mean, if there is no copper available in the atmosphere, how can the galvanic reaction occur?

One could look back in history about the use of iron and copper in ship building, first as nails and later as cladding or covering over the wood, and then as the main structural material, especially for propulsion shafts (most likely steel) and propellers (possibly bronze and less likely a brass). Copper would most likely be in the form of a bronze, but also possibly as an impurity in steels and stainless steels.

https://www.nxymarine.com/blog/the-main-materials-for-marine-propellers-and-the-differences_b22

https://www.copper.org/applications...206-copper-alloys-for-marine-environments.pdf

Lastly, how would the process be accelerated if the experiment were repeated near the coast?

Certainly the closer to the ocean, the more moisture/humidity provides for damp conditions. At water's edge (tidal zone, splash zone), metal surfaces can be wet some or most of the time, depending on how far horizontally and vertically the metal surface is from the sea-level surface.

https://www.wbdg.org/ffc/dod/cpc-source/waterfront-coastal-structures-knowledge-area

Sea spray is another factor. Salts in solution can be carried on the wind and deposit on metals, which over time build up layers of salt and oxides from corrosion. Even when the spray is absent, the layers of salt and oxides are subject to moisture from rain and importantly from condensation of moisture in the air. One may investigate deliquescence, the process of becoming liquid as a result of absorbing moisture from the air. Salts and porous oxide will adsorb water from the atmosphere as well as experience condensation or precipitation (rain), and so go through cycles of wet and dry, with the corrosion rate increasing during the wet phase. This is also a factor in northern climates around lakes and rivers, especially where salt may be applied to highways and roads.

 

[Tom.G]

From an older post of mine: https://www.physicsforums.com/posts/6647026

Somewhat Relevant Story
I'm within walking distance of the Pacific Ocean here. A few years ago the power company had a somewhat similar problem in a 3 block area. It turned out that some splices on the 16kV 3-phase lines (that's the primary side of the consumer distribution transformers) had corroded. It took them three tries to find all of them. they would repair one set and several hours later some splice further up the line would fail.

(Customers too were getting... shall we say... a bit annoyed.)

A conversation with one of the repair crew revealed that the original splices, (saddle clamps) which were Aluminum, are now replaced with Copper.

Cheers,
Tom