Cathodic Protection

What you are seeing is probably localized galvanic corrosion. Current is flowing within cells at each bonding location. The best way to stop it is to isolate the metals with an insulator.

If you were to impress current, you would only change the material that is being sacrificed. The reason cathodic protection works is because one metal is being sacrificed; you put magnesium in the soil with the knowledge that it has a higher potential and will corrode faster when in an electrolitic soil and bonded to a steel pipe.

For cathodic protection to work and protect both of your metals, you would need to add a third sacrifical metal into the circuit, but that probably wouldn't help either because it would corrode and you would loose conductivity at the corrosion point (it works in soil because the anode is bathed in electrolyte.

Drew K

Do they salt the roads there in winter?

Cathodic protection won't help, if they do.

You just need to have the car manufacture electroless nickle plate the steel.

electroless nickel pdf

But then they would not be able to sell new cars to folks like me that will fix and maintain them till they rust out. It's not in the auto manufactures interest to have rust proof products.

Internet auction sites (names withheld) are a great way of disposing of problems like that.

Haha, true but I like the car. I honestly can't see myself buying any other car that's not a BMW 530d station wagon because what else can you put a bed in the back, drive 220 kph, has a little oil fired boiler that preheats your car in winter, has self levelling suspension and is rated for towing 2 tons? As you can see, I'm infatuated with this car haha :D

Problem is, to get one significantly better I'd have to spend about 8000 to 10000 euros in Germany. Another problem, I've replaced a ton of parts (transmission all new brakes and many suspension components). So I figure since this car isn't worth much in Sweden due to cosmetic issues and high miles (worth maybe 3000 euros) I can more or less justify spending money to fix the car. If I spent 5k euros fixing the car, it would be in better condition than the one from germany. Then, at the same time I wouldn't have to go through the hassle of importing a car from Germany (import again, as I had just moved here 8 months ago, it's crappy process!).

Sentiment understood. I own a 3-series 3.0i sedan, but I did test-drive the 530d. What a pull-away! One gets very spoilt, and any suitable replacement is very pricy.

Wish you well with this project.

Thank you! Yeah, it's hard to beat! I mean how much does a new one cost? 60k euros? And there aren't many of these 530d models left in good condition. This was the world's fastest diesel production car when it was new and the world's most economical luxury car :)

Drew, good point. That's what I was wondering, if it would in principle need to be submerged in an electrolyte like pipes would be. Maybe if something could be regularly serviced and replaced each winter season? Magnesium is fairly cheap.

Yeah, the third material is what I intended to ask about. A block of magnesium somewhere on the chassis. Or many small pieces clamped on various places. But from things I've seen on the internet, apparently they do this on tall steel structures and depending on if the potential is large enough, they will add a power supply to increase the potential. I don't know, maybe that's one of the crap products only expensive and for large buildings, but maybe it actually works.

Kramarat, they do salt the highways but generally not the small roads here. It's far too cold for salt to do anything useful for 90% of the winter. Cathodic protection apparently works in sea water as it's used in oil platforms. I imagine it's because the salt creates many local electrochemical cells.

ignator, yeah I completely agree. BMW in this case has really seemed to know how to design a car to last 6-10 years without issue. I doubt it had any significant rust until it was nearly 10 years old when I got it. Now it's 14, and some of the rust concerns me. I don't know what that electroless nickel is, but I'd guess it's something that you can't apply to an already painted and rusted car. Still an interesting article you posted.

Very good explanation and a good answer.

I think the gentleman from Sweden should buy a new car as there are good cars being exported from Sweden. I guess this is a polite answer to him that Change is the charm of life and go get it. Start with a new car and enjoy trouble free lfe (rest of it) and worry about this new car after about 20 years.

It does not have to be a BIMWA (as people here in USA, sometimes. call BMW affectionately) and there are equally good cars as BMW.

In US, for 30 years I never bought a foreign car then I bought a Nissan Quest because they were the only people offering bench seats (because my wife wanted bench seats) and I liked it and fell in love with it and wanted to keep it longer then 13 years (they were trouble free, yes usual maintenance was provided). Sorry I lost it in a fatal accident. It was fatal for the car and was totaled. Then I bought recently, 3 months gao, a Toyota, Hoghlander because they gave 0% interest for 5 years. I hate it because of the rear seat but performs well and wife liked the color of the car and family out voted me. The point I am trying to make is the time and money and efforts you are putting into is not worth it. Enjoy your life and get going with the other car, I am not suggesting which one but go with the flow and spend more time and mony with your family and not with that damn car, the junk.

I know it may be off the subject but I wanted to give you a poiite and a matter of fact answer.

Regards;

Wow! I have heard the name McCready, but had no idea that this was his scam!!!

I have mainly self taught myself what I know about cathodic protection so I may abuse some of the terminology (but I think I have the basic concept down pretty well). I could use some more learning about the chemistry involved but I think I get most of it.

So, what is the process of steel rusting in the air. I had thought that was oxygen from the atmosphere binding with surface molecules of iron to form iron oxide?

Drew K

Any plate (or pipe, etc) of steel is VERY inhomogenous, electrically-speaking. It is LITTERED with 'gazillions' of anodic-and-cathodic sites across its surface (and within).

Corrosion ONLY occurs at the interface with the atmosphere, which, containing *some* degree of moisture as well as numerous pollutants, makes an adequate electrolyte for said corrosion to occur.

A 'local-cell-site' that is more electronegative will give-up an electron readily to an adjacent site that is more electropositive. That site where the electron is gained will now be willing to 'give-up' that electron; but, as happens more-often-than-not (either in atmosheric exposure, or, offshore within the "Splash-Zone") an O₂ atom in the vicinity will ITSELF be/become REDUCED (the Reduction-half of the Corrosion / REDOX reaction), on that surface to which it becomes attached.

An interesting point to mention is that the ANODIC reactions can ONLY occur as fast as the CATHODIC reactions can (or DO) occur.

The more ions there are (or, the greater the concentaration of ions) in the electrolyte, the faster the *corrosion* can occur.

SO ... Question ... how fast does corrosion occur when a thin piece of carbon steel is placed into a beaker of nitric acid? (How concentrated are the hydrogen ions in the acid?) Persons who have witnessed such a mecanism at work without understanding what is happening might EASILY be led to believe that 'acids literally EAT metals', when in fact, all that is happening is that the otherwise 'natural' corrosion rate is being sped-up gazillions of times! Corrosion is a fascinating science unto itself. When understood thoroughly, answers to questions such as WHY can we store nitric acid in (302/304) stainless steel drums, and why can't we store hydrofluoric acid in glass, etc, become 'second nature' to us.

Understanding polarization versus passivation, 'pickling' processes, acid dewpoint corrosion, etcetera should be mandatory-basics for engineering students these days (at LEAST the concept of knowing WHEN they need to Look-Into such phenomena...!)

Adding-to my prior post (pertinent-to the OP) --- within this science, don't forget that there are *4* means of combatting corrosion (of 'structural-steels' anyway) ---

1) Isolation (includes coatings) 2) Inhibitors 3) CP 4) Environmental Control

Of course, there are "variations" on each of those 'main themes'... Good engineering practices trumps 'stupidity' (i.e., lack of education) every time.

Thank you for the good explanation. That clarifies things better, and reminded me of some things from chemistry class in 2005 :)

So the consensus is that there is just no way of making a cathodic protection device for a car, just because in principle it can't work.

"...there is just no way of making a cathodic protection device for a car, just because in principle it can't work" ... 'correct', it cannot work for A CAR (on dry land).

CP works very well on those structures on which it is correctly applied.

For CP to work, you must be able to deliver sufficient cathodic current to meet the structure's cathodic current demand.

Sacrificial anodes on a submerged pipeline or offshore platform literally "pour" (or 'push', due to the Gibb's free energy in them, over and above that of the steel) trillions of electrons into the structure, which 'race-throughout' in order to eradicate those 'disparities' between adjacent (anodic/cathodic) local-cell sites throughout the steel.

This can only be accomplished (ironically-enough) because of the presence OF the electrolyte ... (which ITSELF is the source of the STEEL'S, otherwise, corrosion-issues!)

What happens is that the "Mechanism-of-Corrosion" gets SHIFTED..... FROM the steel TO the zinc/aluminum/indium (used to use mercury, to maintain activity) anodes.

MISTAKES get made when somebody gets 'wise' to the concept of ICCP (Impressed Current CP) systems....trying to 'transition' the technology to a car, for example.

"Sure", impressed current systems CAN throw the CP current vast distances. But, how do they DO that...? IF you understand the mechanism properly, it becomes OBVIOUS that any attempt to make it function on an automobile is....how do we say it....FUTILE!

That's for the clear explanation. That's exactly the answer that I was looking for. I was basically expecting something like this, but I wasn't sure exactly so it's nice to have that clarified.

Thank you for the reply, that is exactly the kind of response that I was looking for.

So I guess that I need a water pool to store my car in :p

So you're saying that the cathodic protection only works over an entire structure if the whole structure is in electrical contact with an electrolyte and it doesn't work in air because air is non conductive? I guess I don't know why that is exactly since steel is a better conductor of electricity than salt water is. I guess it's because in water, the potential difference can be such that the iron won't give up an electron and won't oxidize. I guess it's not possible to replicate that in air?

Yeah, no coating is perfect. I need to do some tests to see which coatings work best. I imagine it would be some combination of good preparation via sandblasting, then zinc spray, a good primer and then a tacky underbody coating to keep it sealed. For body work, not very much seems to last more than about a year or two at best. The only option is to cut it out. There are also lots of "rust converter" products but I have yet to see one that isn't garbage.

The thing that no one has mentioned up to this point is that even though there is a galvanic potential between Aluminum (alloy) and steel, as long as both are coated with a good primer and paint, there is to be expected to be little to no current flow. Salt will not cause the steel to corrode even when bare, as long as a good connection is made to the Aluminum, however the aluminum should suffer some pitting under bare conditions. Magnesium would tend to protect the aluminum, but also a good primer coating, good paint, and a treatment for under the car known as "under-coating" would also tend to reduce corrosion from road salts. You should try some experiments with salt spray (hot box testing) with (1) the steel, (2) steel bonded to aluminum alloy (same as the car), and (3) steel bonded to aluminum bonded to magnesium.

Caution: Magnesium and water together always produces some hydrogen, and if in a confined space will produce steam pressure with hydrogen, also magnesium exposed to any intense flame will ignite and will not be extinguished by "normal" extinguishers.

In any situation be aware that the solution might actually introduce more problems than it solves. Paint is cheap.

In the pipeline fuel industry, it is well known that no coating is perfect. There will always be holidays (holes) in the coating that will allow galvanic corrosion. On a vehicle, rock chips and flexing of the metals will cause cracks in the paint. Underbody coating is nice and thick so it takes rocks with slight deformation and some can actually re-seal if damaged.

The only true solution to a corroding vehicle would be to have it made from a 'stainless' metal.

Very good advice on the hazards of magnesium. I knew of a worker in a scrapyard who was sent to casualty because the 'aluminum' bike he was cutting with a torch caught on fire...then he tried to put it out with water!

I did not know magnesium would crack water into hydrogen and oxygen, is this due to the electrolosis of dissimilar metals or some other reaction?

Drew K

Magnesium is a very active alkaline earth metal. It is well known that it will produce hydrogen and heat up, since the reaction with water is highly exothermic. It should not catch on fire spontaneously as does sodium and potassium.

Tokyo Institute of Technology recently announced plans for a magnesium fuel cycle, where the steam is produced, and the hydrogen burned to produce more and hotter steam to drive a turbine (or a piston engine). The back half of the cycle converts magnesium oxide directly back to magnesium (and oxygen) without the use of silicon (as in the well known Chinese process) by using a solar-pumped Cr-Nd YAG laser (of remarkably high solar efficiency). Exactly how the Japanese plan to separate the Magnesium vapor from released oxygen is a mystery to me, but I have seen low detail cut-away showing this reactor concept. My mind can't get around several things about this: (1) it takes a temperaure higher than can be reached with direct concentrated sunlight, yet the laser can achieve this (apparently MgO absorbs 1.06 micormeter wavelength laser light very efficiently), and (2) that there is not instant recombination of the magnesium and oxygen just outside the hot zone.

Perhaps the trick is that the oxygen atoms very rapidly form oxygen molecules (quasi-stable) and that the auto-ignition point of magnesium vapor with oxygen must be also an astronomically high temperature for some unknown reason.

I hate to bring up this old thread, but someone try to explain this! I have an old car battery sitting around just in case I need to give someone or myself a jump. It has been sitting completely disconnected in a dry environment on my apartment floor. I put some brass cable clamps on there because I had them sitting around. I put those on there less than a month ago and they were identical and brand new with no oxidation on them.

Negative terminal (ignore the + on the clamp, it is the negative pole):

Positive terminal:

I've never seen such severe oxidation so quickly on something completely unused. Then I notice the positive terminal has absolutely no oxidation on it whatsoever. It's not just the interfaces that are severely oxidized on the negative terminal, it's essentially all the surfaces on the brass and galvanized bolts. The bolts are extremely oxidized. The zinc is essentially gone and comes off in a hard caked powder.

Isn't this the opposite of what you'd expect? With impressed current cathodic protection, the positive terminal would oxidize. Also, there's clearly no current flowing, aside from what goes through the humidity in the air which is extremely small of course.

I'd really appreciate some explanation! I'm really curious what caused this.

Assuming all else is the same, it looks like there is some minute gassing off happening around the negative terminal. It doesn't take much.