I am trying to learn about 1930’s steel aircraft construction. The more I read the more I think that sacrificial magnesium anodes should be stuck to historical aircraft as good preservation practice.
Lots of things I have seen : historical aircraft parked in the open, corroding as they wait for a lottery win, well meaning static restorations using dissimilar metal rivets from a hardware store, purposeful use seventy years ago of dissimilar metals in what are now vintage aircraft originally designed to last not much longer than a modern battery hen and modern engineering inadvertently incorporating predictable, future failure from the use of components setting up galvanic corrosion, caused by folk like me before I started reading old aircraft engineering books…
Google galvanic corrosion to understand why Old Jack told me about picking magnesium rivet heads off a Spitfire wing with his fingernail. I remember now working on old steam ships and wondering why they were so mad about renewing sacrificial zinc anodes on 100 year old steel hulls, and now I realize why these historical ships will last another hundred years while lots of aircraft won’t. The seafaring folk have a really good understanding of this issue and could really help transfer some of this knowledge to historical aviation. By golly we could even stick a lump of zinc to the falling hangar that can’t be restored for lack of funds.
Basically a sacrificial anode on a steel or aluminium aircraft could be a small, replaceable disc of magnesium fixed to a visible and accessible part of the structure, perhaps a wheel well, even the cockpit. Such a disc would cost a few dollars and would forgive a multitude of sins : storing of aircraft in the open, originally specified reactive metals and new reactive metals inadvertently incorporated in restoration. There are thousands of parts in an aircraft and it is impossible to isolate them all from electrochemical reactions or build an understanding of their metallurgy. Even a simple thing like a magnesium wire specified for the attachment of identification labels on stored parts could slow them rotting away on the shelf.
I figure that the engineers of the 1930’s did know about galvanic corrosion, and the influence of a century of ship building knowledge, whether by being in the air of the time, or by direct connection with the building of flying boats, made this design consideration visible in metal aircraft engineering in the 1930s. I figure that where they could, they chose materials with electrochemical compatibility. In the Hawker biplane of the 1930’s, the metals chosen for wing spars, wing ribs and a particular stainless plate were remarkably compatible in electrochemical potential, and I see these parts in reasonable condition 80 years later. The same stainless plate acted as cathode to the anode of the steel tubing in the fuselage of the same aircraft, and I have yet to see fuselage tubes I could use to comfortably prop up my car.
I figure that in the expediency of war, where an airframe was expected to last for hours not decades, this compatibility was a lessor priority. Surely they knew that magnesium rivets would rot in an aluminium fuselage, but they could not imagine that mad men and women would want to fly in these mechanisms 70 years later ! But we do. The real problem is that there is no quick accountability for the affects of galvanic corrosion. Our poor decisions will be a problem for our grandchildren to ponder when historic aircraft turn to powder. Maybe they will have other things to worry about, paying off our debt as they paddle over rising seas.
If there is a desire to see these amazing flying machines last for 100 years, this is a very demanding design factor for a metal object. Where there is the unction to adhere to original engineering specifications, this could be utterly contrary to this consideration of longevity.
A little magnesium disc. Quiet, unobtrusive, solid, dependable, doing a good job in the background. Maybe we could call it the Denis Thatcher disc…
By: Vega ECM - 23rd June 2013 at 15:20
The finest sacrificial anode protection for steel is cadmium plating. Now that the save the planet guys are trying to ban it the next best is zinc nickel plating. With highly stressed steel parts it’s vital to conduct a de-embrittlement heat treat to drive of surface hydrogen. Both provide excellent area protection to the substrate even if it’s locally exposed. Cad or ZnNi plating should be painted to prevent coating abrasion.
Another steel sacrificial coating which can be applied like a paint is MCAC such as Alochrome
By: powerandpassion - 23rd June 2013 at 12:07
Sweet thought
[QUOTE=powerandpassion;2015981]I am trying to learn about 1930’s steel aircraft construction. The more I read the more I think that sacrificial magnesium anodes should be stuck to historical aircraft as good preservation practice.
By golly I had another thought ! Use sugar as a cheap aircraft preservative ! I am thinking airframes stored in the open, with difficult to access sections where corrosion sets up, where trickling water from condensation may pool and over the years accelerate corrosion. The use of silica crystals as a moisture absorbent is well established practice. But this stuff is expensive, and regenerating the silica requires treatments with dehumidified air or oven. Mainly it’s expensive. Silica absords small amounts of water slowly and releases small amounts of water slowly. You really want something greedy for water that will also release it fairly readily, like a kid with a potato chip.
Take a food technologist out on a date and ask them the explain ‘water activity’ in food science. Ie , why a moist food like jam can sit on the shelf and not go off. Sugar in jam acts as a humectant, binding water molecules to it, making them less available for bacteria to grow, ergo low water activity. I guess the same principle will act for metal. Find where water pools in a static airframe and pour a pile of cheap sugar in the spot. The sugar will grab at water molecules faster than any possible reaction of metal with the water. On a warmer day the sugar will release the water molecules and the cycle can regenerate day -night. I cannot see the formation of any chemical reactions between sugar and metal, that may harm the metal. Salt is also a humectant in its food science application, but will set up an ion pathway for electro chemical corrosion. Maybe some ants will come in.
I will have to think up a test and control to prove or disprove this sweet thought.
So folks, sacrificial magnesium anodes outside and sugar inside, how to save a neglected airframe for five bucks.
I cannot help you with home remedies for getting earwax out of old headphones.
By: powerandpassion - 21st April 2013 at 12:26
Excellent thread, P & P. Very interesting topic. I own a large boat and have recovered aircraft components from saltwater. I have witnessed instances of dissimilar metal touching where one acts as the sacrificial anode and is dissolved whilst the other remains perfectly intact. I’ve seen it go either way, steel disolves and Aluminium remains intact, and aluminium disolves and steal remains intact.(Yes, dural does have Copper and magnesium in it, so is not strictly pure aluminium). However I dont believe your suggestion would have much effect on dry land. Simply because most corrosion on dry land is caused by prolonged or intense exposure of a bare metal surface to oxygen, and I cant imagine that a sacrificial anode would alter this. Better protective coatings would be more beneficial; ie Alodine, Zinc Chromate, Primer, Paint , fabric covers, shade, a hangar, a climate controlled museum building,etc…
I once owned a P-47D Razorback canopy that was recovered from a wartime airstrip roughly 300 metres from the ocean, the canopy had been lying upright alongside the airstrip for 40+ years. The side of the canopy facing the mountains was in very good condition, with surface paint intact although very faded. The side of the canopy facing the ocean, was bare metal and covered in very small corroded pitting. The constant sea breezes had (along with dust and sand, no doubt) removed the protective covering, and the constant flow of air against the bare metal had caused significant corrossion. I would agree that the breeze probably contained a reasonable quantity of NaCl (salt) as well which would not have helped.
I guess the closely packed molecules in liquid is better for electrolysis than the widely spaced molecules in a gas(air). Great suggestion though, had never heard it before.
Mate, thank you, you feel like a bit of a mug posting sometimes so I appreciate a positive response.
I agree with the sentiments about proper storage, I guess the reality is that most objects are kept in improper storage for lack of funds. Most of the paint systems modality, eg zinc based, is to be a sacrificial anode, so a reasonable thing to do is cover an airframe with zinc based paint, but I figure this is more expensive and time consuming than adding a small disc of magnesium.
Intergranular corrosion of duralumin and other alloys is a fascinating or frightening thing. Its like a whole lot of atoms with boxing gloves punching each other. The old motorbike blokes complain about their crankcases turning to powder, despite being garaged and loved, and bathed in oil. My understanding is that the cheap, poorly formulated aluminium alloys found their way into the motorbikes, with higher specification, more expensive aluminium used for aircraft. Theoretically. Sometimes you look at an alloy cased aero engine accessory and it is turning to powder, while other parts are not. I guess one way to save money was to use the cheap alloys, who would care about it 50 years later ? I wonder how the Japanese material you stumble across looks in comparison to the American stuff ? My understanding is that bauxite sources to the Japanese were pretty much cut off by 1944 so that they must have alloyed their aircraft material with sawdust to make it go round. Perhaps they just melted up the odd B-29 that came down to keep the show on the road.
The more you get into the metallurgy the more witchcraft, magnificence and shoddiness you see in design using metals. Certainly we live in an age orbited by shoddy oriental metallurgy where it is normal to buy a power drill for $20 bucks and then shrug when the chuck splits. When it comes to historical aircraft the metallurgical sins of the past are now apparent. I guess the sin of the present is to know and do nothing about it.
By: 43-2195 - 20th April 2013 at 11:51
Excellent thread, P & P. Very interesting topic. I own a large boat and have recovered aircraft components from saltwater. I have witnessed instances of dissimilar metal touching where one acts as the sacrificial anode and is dissolved whilst the other remains perfectly intact. I’ve seen it go either way, steel disolves and Aluminium remains intact, and aluminium disolves and steal remains intact.(Yes, dural does have Copper and magnesium in it, so is not strictly pure aluminium). However I dont believe your suggestion would have much effect on dry land. Simply because most corrosion on dry land is caused by prolonged or intense exposure of a bare metal surface to oxygen, and I cant imagine that a sacrificial anode would alter this. Better protective coatings would be more beneficial; ie Alodine, Zinc Chromate, Primer, Paint , fabric covers, shade, a hangar, a climate controlled museum building,etc…
I once owned a P-47D Razorback canopy that was recovered from a wartime airstrip roughly 300 metres from the ocean, the canopy had been lying upright alongside the airstrip for 40+ years. The side of the canopy facing the mountains was in very good condition, with surface paint intact although very faded. The side of the canopy facing the ocean, was bare metal and covered in very small corroded pitting. The constant sea breezes had (along with dust and sand, no doubt) removed the protective covering, and the constant flow of air against the bare metal had caused significant corrossion. I would agree that the breeze probably contained a reasonable quantity of NaCl (salt) as well which would not have helped.
I guess the closely packed molecules in liquid is better for electrolysis than the widely spaced molecules in a gas(air). Great suggestion though, had never heard it before.
By: powerandpassion - 20th April 2013 at 03:56
closed circuit
For this to work there must be an electron pathway between the anode and the metal to be protected (e.g., a wire or direct contact) and an ion pathway between both the oxidizing agent (e.g., water or moist soil) and the anode, and the oxidizing agent and the metal to be protected, thus forming a closed circuit; therefore simply bolting a piece of active metal such as zinc to a less active metal, such as mild steel, in air (a poor conductor and therefore no closed circuit) will not furnish any protection.
The closed circuit is the fuselage, moncoque aluminium or in the 1930s steel girder type frame and what ever is bolted or riveted to it. If it wasn’t a closed circuit then all the wiring services in these aircraft couldn’t be grounded to the airframe. Only the wooden mosquito has twice as many electrical cables as a metal a/c because of the need to provide a ground return that can’t go through a timber fuselage.
The ion pathway is the rain, fog, hail, snow, salt spray in open storage or condensation that forms going from cold heights to warmer saturated air at the ground in a flying aircraft. Even an a/c in a climate controlled museum can be affected by breathing humans, as it is for ancient cave paintings or egyptian tombs visited by tourists. Most well funded museums will monitor anthropomorphic ambient humidity.
The fixing of sacrificial zinc anodes to steel ship hulls is standard practice in maritime conservation and modern shipping : it works and I have enjoyed walking on the decks of century old vessels courtesy of it.
Probably very few really think of electrochemical corrosion in modern engineering design let alone historical aircraft preservation, yet it is utterly preventable, and cheap to do.
The use of magnesium wheels on aircraft from the 1930’s onwards provides an additional benefit in being a sacrificial anode to the rest of the airframe, which explains why they have not lasted like the airframe above them.
OK so what I will do now is invest two bucks in a sacrificial anode understanding that as everything else corrodes away the value of what is protected will rise as it becomes rarer and rarer….
By: Rockhopper - 19th April 2013 at 18:01
For this to work there must be an electron pathway between the anode and the metal to be protected (e.g., a wire or direct contact) and an ion pathway between both the oxidizing agent (e.g., water or moist soil) and the anode, and the oxidizing agent and the metal to be protected, thus forming a closed circuit; therefore simply bolting a piece of active metal such as zinc to a less active metal, such as mild steel, in air (a poor conductor and therefore no closed circuit) will not furnish any protection.
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April 19, 2013 at 3:56 pm