Rust Removal

Many steel items in the workshop tend to acquire rust on the surface. These are either tools, projects under construction which have been stored for a while, or items from that useful pile of 'junk' stored in the corner or under the bench. Unless extreme or time consuming methods are used for protection, some rusting will always occur. But how do you remove this rust? and hopefully with minimum effect on the item being cleaned.

Rust arises from the surface oxidation of iron or steel in the presence of atmospheric oxygen and moisture. As the rust forms, the surface of the iron is eaten away and converted into rust, sometimes evenly, but often deep local pitting occurs beneath wart-like protuberances. Rust occupies a larger volume than the iron it replaces, and so moving parts will tend to seize as they rust. Chemically, most red rust is hydrous ferric oxide FeO(OH).

Methods of removing rust

There are several ways to remove rust from steel items:
  1. Mechanical removal
  2. Acid (chemical) removal
  3. Electrolytic removal

 

1. Mechanical Removal

This is perhaps the most basic of methods, simply scraping by various means to remove the rust. Although simple, it is very effective, relatively easy to do, and quite often adequate for the job in hand. Best suited for projects that are not too dimensionally critical or cleaning up a 'stored' piece of raw material requiring further machining. The various methods include wire brush (either hand brushing or powered brush), sand blasting, needle gun, sand/emery paper, filing or machining off, even an angle grinder, etc.

Advantages: minimum equipment required, relatively safe, relatively fast.
Disadvantages: Produces the most damage to the item, dimensions of the article may be altered, difficult to get into small crevices and other inaccessible locations.

 

2. Acid Removal

Another method is to use an acid bath to convert the rust (iron oxide) chemically into another form, which can then be washed away. Many different acids can be used including hydrochloric (also known as muriatic) acid, phosphoric acid, citric acid, acetic acid (vinegar), etc. Phosphoric acid is available from paint shops, and hydrochloric acid from hardware stores (it is used to clean new brickwork and cement). Citric acid is available in powdered form from the supermarket (used in cooking). The strength of the acid is in the weaker range. Concentrated acids are too strong, and produce dangerous fumes.

Method:
  • Mix up the acid solution in a plastic bucket or other suitable container outdoors. And remember to only add acid to water, not the other way around.
  • The article from which rust is to be removed should be free from grease and oil as far as possible.
  • Acid will not strip paint, so remove any paint first.
  • Place article into the acid bath. Bubbles will start rising. This is normal - No bubbles equals no or little action.
  • As the process is happening and rust is converted into other forms, it turns into 'gunk' which may need to be removed occasionally.
  • Stirring the solution may help give a faster and more even removal.
  • When rust is removed, rinse the article under running cold water, and use a scourer if required to remove any accumulated gunk. You may wish to neutralise with a baking soda solution, but is rarely necessary.
  • Wash the item under fresh water, then dry it. Placing in a low temperature over, or outdoors in the sun removes any remaining water.

Now that you have a cleaned and dry article, is will be more susceptible to acquiring a new layer of rust. Either paint it for protection, or smear oil if desired.

Notes:

  • Leave the article in the bath for as long as there is still rust on it, and bubbles are rising.
  • Using a warm or hot (not boiling) bath will accelerate the process.
  • The stronger and hotter the acid bath, the quicker it works. So pay attention and watch - rust removal may only take minutes.
  • Remember the acid keeps working after the rust is dissolved, and will continue to etch the steel. This will result in the surface being covered in microscopic etched pits giving a dull grey appearance. So do not just leave the article in the acid bath for long periods.
  • Acid rust removal is not a 'chuck in and forget process'. Think about the outcome you want, as well as the state of the article. Adjust the concentration to the task - light concentration for lightly rusted items, heavy concentration for encrusted items; AND keep an eye on what is happening; AND take article(s) out regularly to clean off the gunk.
  • When completely finished, dispose of the used acid bath in an environmentally thoughtful manner.

Advantages: Very thorough process able to work into the smallest of spaces, leaves the surface clean for further processes (e.g. painting), little or no damage to underlying metal, relatively fast.
Disadvantages: potential danger of working with acid, produces corrosive fumes and requires outdoor ventilation, possible disposal issues of used acid bath.

 

Safety Tips when working with acids

  • ALWAYS add acid to the water, NEVER the other way around when mixing any acid solution.
  • It is always a good idea to wear eye protection.
  • It is recommended to wear gloves
  • Adequate ventilation. Do not use this process indoors - set up outdoors where there is plenty ventilation, and no risk of any fumes inducing rust in other items or tools in the workshop.

 

3. Electrolytic Removal

The electrolytic method is an effective, cheap(!), gentle and effective method of rust removal which causes minimal alteration to the metal surface of the article to be cleaned. It may seem complex, but is easy to set up and use. It does not damage the underlying material, and is frequently used in restoration work of old objects.

More details can be found in this article on electrolytic rust removal as the description is too long for this page.

Advantages: Very thorough process able to work into the smallest of spaces, leaves the surface clean for further processes (e.g. painting), no damage to underlying metal, safer than acid baths, cheap, impossible to 'over-clean', can be used indoors.
Disadvantages: more complex setup.