This is just a list of what I know and use, how it works an the effects you get.
First of all. Iron can form two types of oxide, red and black. Red rust is soft and "active" - will continue to rust if not treated, and black is more or less inactive if you can seal the water out. Better than this is no rust at all ! Red rust contains more oxygen than black rust.
Mechanical Removal
Always the first step is to remove as much as you can with a wire brush, grinder, flap wheel, hand brush, or a metal pan scourer works quite well. Emery cloth is usually the next step to get into awkward areas. If I had a grit blaster I would use that, but I don't, and neither do most people.
Reach for the Jenolite
- Is what most people do, but is moderately expensive. It was developed by the US Navy and is mostly Phosphoric Acid and hydrated silica to make a jelly paste. The acid reduces the iron(3)oxide to Iron(2)oxide, then to just iron, with the oxygen reacting to make water. The acid reacts with the surface of the steel to make Iron Phosphate - which is a very tough coating that is ideal to protect the steel. It is slightly darker than clean steel and has a glassy appearance.
Phosphoric Acid
This is what I use from a commercial suppliers in a 5L container, and is very thin, so tends to run off. To combat this I use strips of kitchen roll on top to form a type of poultice to keep the acid where I want it.
It is quite cheap. Several applications are usually needed to remove rust out of pits, and sanding helps between uses.
Citric Acid
This is the same citric acid used in cooking, and on fizzy sweets. It is edible and non-toxic. It is also an excellent derusting fluid for immersion. I make up a jar of it, and use it to remove rust from bolts after wire brushing.
It's not actually an acid per se, but removes oxygen by Chelation. The results is the same - no more rust. But if there is any zinc still left on the part it will leave the zinc intact.
The result after an hour or so is that the rust looks like it is still there, but has softened so that it can be wiped away, leaving clean metal underneath. Rinse with water, dry and spray with cold galvanising paint to restore your bolt to almost original condition.
It does have a bad habit of secondary flash rusting, and dulls quickly in air, so to combat this I dunk it in tea and wash it in that instead.
Why Tea? - It is a very strong solution made by boiling old tea bags till it is a thickish liquid that is high in tannin. The Tannin combines with the steel to form a compound that prevents further rusting. This is the "Tan" in products like "Fertan" or "Vactan" that are sold commercially. For all I know maybe they are just citric acid and tea.
Citric acid can be used on anything you can keep wet or immerse, is quite cheap and non toxic.
Electrolytic De-rusting
This is where you pass a small current through the component to remove the rust.
I use Washing Soda in hot water, about a tablespoon per bucket is enough.
Attach your component ( I almost said parts but could prove painful) to the NEGATIVE of a small 12v maintenance charger and place it in solution. Attach the POSITIVE to a scrap piece of steel (which can be rusty as long as the connection is good) and place that in solution to face the part being derusted. Think "Sacrificial Anode" and you will remember which way round it goes.
Bubbles rise at both parts, but your rust will disappear from the bit you want, as the anode steadily becomes more rusty!
What is happening is that hydrogen formed at the anode is combining with the oxygen in the rust - leaving just the iron behind, which is physically smaller. The result is clean parts covered with soft black iron and red rust that is not electrically 'stuck'. Both clean off easily. with a wire brush.
There is a small chance of "Hydrogen embrittlement" in high tensile steels, so should not be used for parts such as brake discs or calipers, or vanadium tools, but for most things is very very cheap and very effective.
It works better on items with some paint left, as the paint acts as an insulator for most of the good metal. Only the rusty metal is worked on.
Again flash rusting can be a problem once you take it out, so rinse with tea solution or a quick brush with phosphoric acid will prevent this.
Coatings
Next you need to protect your steel (when dry)
Cold galvanising spray is good for nuts and bolts or something needed quickly as it is a thin coating and dries fast.
Zinc Phosphate primer is I think the best as it fills the rusted areas and the adhesion bond is excellent for further top coats.
Hammerite style paints do work but I find they chip a bit too easily, but a good quick finish coating if sprayed on. I use a smooth black satin paint for a lot of parts.
Conversion primers
These are paints that say they can kill rust and prime in one. In my experience they only work when you have already converted the red rust to the more stable black oxide form. They seem to work by excluding moisture and therefore oxygen from the surface - therefore no more rust can form. Just don't paint them on red rust or you are wasting your time.
I have used them on large areas of chassis after I have removed as much as I can with mechanical methods then phosphoric acid, leaving just black oxide.
I am using Flag Primer at the moment which is black. Sometimes it works, sometimes it doesn't and the rust pokes through again.
Piccies to follow.
First of all. Iron can form two types of oxide, red and black. Red rust is soft and "active" - will continue to rust if not treated, and black is more or less inactive if you can seal the water out. Better than this is no rust at all ! Red rust contains more oxygen than black rust.
Mechanical Removal
Always the first step is to remove as much as you can with a wire brush, grinder, flap wheel, hand brush, or a metal pan scourer works quite well. Emery cloth is usually the next step to get into awkward areas. If I had a grit blaster I would use that, but I don't, and neither do most people.
Reach for the Jenolite
- Is what most people do, but is moderately expensive. It was developed by the US Navy and is mostly Phosphoric Acid and hydrated silica to make a jelly paste. The acid reduces the iron(3)oxide to Iron(2)oxide, then to just iron, with the oxygen reacting to make water. The acid reacts with the surface of the steel to make Iron Phosphate - which is a very tough coating that is ideal to protect the steel. It is slightly darker than clean steel and has a glassy appearance.
Phosphoric Acid
This is what I use from a commercial suppliers in a 5L container, and is very thin, so tends to run off. To combat this I use strips of kitchen roll on top to form a type of poultice to keep the acid where I want it.
It is quite cheap. Several applications are usually needed to remove rust out of pits, and sanding helps between uses.
Citric Acid
This is the same citric acid used in cooking, and on fizzy sweets. It is edible and non-toxic. It is also an excellent derusting fluid for immersion. I make up a jar of it, and use it to remove rust from bolts after wire brushing.
It's not actually an acid per se, but removes oxygen by Chelation. The results is the same - no more rust. But if there is any zinc still left on the part it will leave the zinc intact.
The result after an hour or so is that the rust looks like it is still there, but has softened so that it can be wiped away, leaving clean metal underneath. Rinse with water, dry and spray with cold galvanising paint to restore your bolt to almost original condition.
It does have a bad habit of secondary flash rusting, and dulls quickly in air, so to combat this I dunk it in tea and wash it in that instead.
Why Tea? - It is a very strong solution made by boiling old tea bags till it is a thickish liquid that is high in tannin. The Tannin combines with the steel to form a compound that prevents further rusting. This is the "Tan" in products like "Fertan" or "Vactan" that are sold commercially. For all I know maybe they are just citric acid and tea.
Citric acid can be used on anything you can keep wet or immerse, is quite cheap and non toxic.
Electrolytic De-rusting
This is where you pass a small current through the component to remove the rust.
I use Washing Soda in hot water, about a tablespoon per bucket is enough.
Attach your component ( I almost said parts but could prove painful) to the NEGATIVE of a small 12v maintenance charger and place it in solution. Attach the POSITIVE to a scrap piece of steel (which can be rusty as long as the connection is good) and place that in solution to face the part being derusted. Think "Sacrificial Anode" and you will remember which way round it goes.
Bubbles rise at both parts, but your rust will disappear from the bit you want, as the anode steadily becomes more rusty!
What is happening is that hydrogen formed at the anode is combining with the oxygen in the rust - leaving just the iron behind, which is physically smaller. The result is clean parts covered with soft black iron and red rust that is not electrically 'stuck'. Both clean off easily. with a wire brush.
There is a small chance of "Hydrogen embrittlement" in high tensile steels, so should not be used for parts such as brake discs or calipers, or vanadium tools, but for most things is very very cheap and very effective.
It works better on items with some paint left, as the paint acts as an insulator for most of the good metal. Only the rusty metal is worked on.
Again flash rusting can be a problem once you take it out, so rinse with tea solution or a quick brush with phosphoric acid will prevent this.
Coatings
Next you need to protect your steel (when dry)
Cold galvanising spray is good for nuts and bolts or something needed quickly as it is a thin coating and dries fast.
Zinc Phosphate primer is I think the best as it fills the rusted areas and the adhesion bond is excellent for further top coats.
Hammerite style paints do work but I find they chip a bit too easily, but a good quick finish coating if sprayed on. I use a smooth black satin paint for a lot of parts.
Conversion primers
These are paints that say they can kill rust and prime in one. In my experience they only work when you have already converted the red rust to the more stable black oxide form. They seem to work by excluding moisture and therefore oxygen from the surface - therefore no more rust can form. Just don't paint them on red rust or you are wasting your time.
I have used them on large areas of chassis after I have removed as much as I can with mechanical methods then phosphoric acid, leaving just black oxide.
I am using Flag Primer at the moment which is black. Sometimes it works, sometimes it doesn't and the rust pokes through again.
Piccies to follow.
