Mill Scale on the joint of the mild steel you're planning to weld is an impurity on the surface of your weld. Along with other possible impurities such as oil, paint and rust, it is best to remove the mill scale before you weld. Particularly if you want the cleanest looking and strongest weld.
Ways of Removing Mill Scale on Mild Steel
1. Dipping in or Spraying on Muriatic Acid
2. Dipping in White Vinegar
3. Grinding Mill Scale off with a Grinder
What is Mill Scale on Steel?
Mill scale is dark grey iron oxide formed as a layer on hot rolled steel.
Cold rolled steel doesn't have mill scale on it but it is more expensive to buy and is not suitable for all applications.
Why Go To The Trouble of Removing Mill Scale?
You remove because:
1. Mill Scale, especially thick mill scale can cause porosity - holes both on the surface and under the surface of your welded joint.
2. Mill scale melts at a temperature higher than the base metal mild steel below.
This difference in melting points gives your MIG welder and its electrical arc a barrier to burn through before melting and welding the mild steel below. Resulting in a bad arc start and depending on the welding output and the gauge of metal you're welding you may find your arc stuttering as it attempts to melt the mill scale covering.
3. Any mill scale will need to be cleaned off your sheet metal before painting after welding.
4. You want to have minimal clean up after welding with as nice clean looking weld as possible.
MIG Weld Through It?
Some experienced welders are happy to MIG weld through a light coating of mill scale.
Should you choose to weld through the mill scale, then look for mild steel solid MIG wire containing Silicon and Manganese. These additions to the MIG wire act as deoxidizers and are tolerant of light levels of impurity on the base metal.
They will collect up impurities and send them up to the surface of your weld as 'islands of silicon' to be chipped off once the weld has cooled.
Ideally, when MIG welding, you're looking for a good clean surface to weld on. Mill scale removal is part of the normal weld preparation to get a quality weld.
Let us now look at the methods of removal. It's worth understanding that whether using acid cleaning or mechanical cleaning with a grinder of mill scale it is best to remove any surface grease first.
1. Mill Scale Removal with Muriatic Acid
Is an option to consider when you have a thick coating of mill scale and or a lot of rust on old steel and you know there is good steel underneath.
Largely using Muriatic Acid to remove mill scale is a dipping process. This means that you will need a suitable, acid resistant bucket or tank to put your project into. Preferably a bucket or a tank with a good lid that can be closed tight.
Needing a bucket or tank implies that that you have or can source a container suitable for the acid and big enough to submerge the part in.
I found this tank from Bel-Art on Amazon. It is a heavy duty polyethylene rectangular tank with a good fitting top cover. It's also a reasonable size.
What is Muriatic Acid?
Muriatic Acid contains hydrochloric acid and can be sourced at 20% or 28% or 32% hydrochloric acid potency.
Muriatic Acid is also used for pool care and can be obtained in a pool supply store as well as on line.
I found this bottle of Muriatic Acid for you - link to Amazon
It is 32% Hydrochloric Acid so it pretty strong. Take Care.
950 ml bottle.
Consider getting along side your bottle of acid some self protection for yourself
Gloves that resist strong acid.
And some eye protection with:
Preparation for Using Muriatic Acid
You are dealing with a strong acidic solution when you use Muriatic Acid to remove mill scale. Great care needs to be taken with your personal protection.
You will need to use gloves suitable for acids, eye protection (goggles) and preferably a respirator. Fumes from the Muriatic Acid are no joke and should absolutely not be breathed in. You will not be thrilled with having hydrochloric acid fumes in your lungs.
Your face, arms, legs and feet need to be protected from accidental splashes of the acid.
Consider covering any exposed skin.
You will need access to a hose and lots of water to wash your clean parts after dipping. Have clean water readily reachable to deal with any accidental splashes.
Mill scale removal with Muriatic Acid should be considered an outside process. The fumes are fierce, I did mention not breathing them in didn't I. The fumes will affect, given enough time and exposure, any metal tools nearby by eating away at the metal.
That's why the emphasis on doing your soaking outside, well away from the house or garage. And then use a bucket or container with a cover. Keep those fumes in.
Your metal will need any oil removed before using the Muriatic Acid. Use soap or a preparatory solution to degrease.
Dipping with Muriatic Acid
Place your bucket or dipping tank on a firm surface.
You may want to do this away from decorative concrete areas if you don't want to risk staining from splashes when you remove your part.
- Pour in the Muriatic Acid.
- Carefully put in your mild steel parts.
- Check progress after 15 minutes. Check again at 20-30 minutes and your part should be nice and clean. 30 minutes total dipping time should be sufficient.
- Remove the parts once the mill scale is removed and wash with clean water thoroughly.
If you have some light mill scale left that can be easily sanded off.
Spraying with Muriatic Acid
This way of using the Muriatic Acid is better for large or heavy pieces of mild steel. Those pieces that can't fit into a bucket or tank.
You will need an acid resistant spray bottle to hold the Muriatic Acid.
I quite like this one from the Chemical Guys, it is a 32 oz heavy duty bottle and you can get extra sprayer heads.
And a location outside where you can place your project and spray safely.
A nice calm day - having the wind blow the acid on stuff or vegetation nearby or over you is not a good look.
- Spray on.
- Check progress after 15 minutes, and again at 20 to 30 minutes.
- Wash off with water once the mill scale is removed and blow dry off your part.
Any light mill scale that's left can be easily sanded off.
Metal Care After Mill Scale Removal
Mild steel rusts amazingly quickly after dipping in the Muriatic Acid and will need protection if you're to stop it rusting straight away.
Consider neutralizing the acid, as this will slow the rusting. Spray with Windex containing Ammonia.
Or make up a mix of Washing Soda or Baking soda with water. Mix one gallon of either in water to one cup of baking soda and scrub the metal.
After neutralizing an oil covering is probably a good idea so that you're not undoing the good work you've done cleaning up the steel by leaving it exposed to the air to rust.
Leave the joints your planning on MIG welding clean if you are going to weld straight away.
Disposal of the Muriatic Acid After Use
Please take care to dispose of the acid safely after dipping, use lots of water to dilute and neutralize the acid. Always add the acid to water and use lots of water to dilute the acid until it is completely neutralized.
If you have left over acid in the bottle you would like to use another time, then store it somewhere safe in its original container, with the cap firmly on.
And for safety it is probably best to store side outside the house, say in a garage just to be ultra safe. Up and away from curious little hands or pets. Please handle Muriatic Acid with respect.
Reasons to Choose Muriatic Acid for Mill Scale Removal
Depending on the size of the parts you want to weld, the removal of the mill scale is relatively quick taking 15 to 30 minutes.
Reasons to Stay Away from Muriatic Acid for Mill Scale Removal
The whole dipping process needs to be done outside, which means having suitable weather conditions to do it in. If you live somewhere with cold winters then it may be something you can only do at certain times of the year.
You need a safe space where you can do your dipping or spraying.
If you have children or pets roaming around or even if you are the clumsy sort do you want to use and have strong acid around?
You may have a medical condition where breathing even the tiniest amount of the fumes is unacceptable.
Dipping in Muriatic means that the part you're cleaning is small enough to fit inside the bucket or tank you are using. This may not be the easiest of things to do if the part is large or heavy.
Some will find the safety aspect and getting dressed up to use the Muriatic acid a real pain and fuss. Yes, the use of the acid is quick but by the time you have added the preparation, safety clothing and neutralizing of your metal, it may have taken you longer than you thought. Plus there is the whole disposal and storage to deal with.
You'll see the reasons not to use outweigh the reasons to use. This does not mean it is not a good solution. With the right precautions and in the right situation this could be a great solution for getting rid of that pesky mill scale.
YouTube Video Showing the Use of Muriatic Acid for Mill Scale Removal
I found this video on YouTube, he demonstrates the results you can get through dipping mild steel into Muriatic Acid. Note he uses gloves but is doing the dipping in shorts! Not something I would recommend. Ignore that, as it is a good video that will give you a good idea on how it all works.
How to Chemically Remove Mill Scale
Video Credit: Ramsey Customs - turbocobra
2. Mill Scale Removal with Normal White Vinegar
This is the cheapest solution if not the quickest method of removing mill scale before MIG welding. Similar to Muriatic Acid use this is a dipping or spraying on method. White vinegar is less acidic and so overall safer to use.
You will need a bucked or a tank suitable for containing acid. Yes vinegar is not quite as strong but that does not mean it won't eat through a container given time. A lid that can be closed it good as it just keeps the vinegar smell contained.
What Vinegar to Use
Any normal household white vinegar can be used, sourced at your local market, Wall Mart or on line.
I like this one from Amazon 30% Pure Vinegar - Home and Garden 1 Gallon.
Preparation for Using White Vinegar
A good pair of gloves suitable for acids. If you use your hands that white vinegar will seek out any cuts of scratches you have and they will sting like hell. Perhaps consider using goggles because a splash of vinegar in the eye will not be pleasant.
Dipping with White Vinegar
Unlike Muriatic Acid white vinegar can be used indoor or out. It just takes more patience.
- In your chosen bucket or tank pour in your vinegar.
- Put in your mild steel project.
- Leave overnight or more likely for 24 hours as it will take sometime for the vinegar to work.
- Put on and tighten the lid to contain the smell of vinegar.
If you are putting a number of pieces of low carbon steel in together then you may need to turn them over part way through the soaking time. This will give the vinegar the best chance to get to all of the parts.
Don't forget your steel is soaking or be tempted to leave the parts in the vinegar for days without checking. After all you are using an acidic substance and vinegar will eat away at the steel if left in too long.
Once the mill scale is removed, then wash or hose the parts off with water to neutralize the vinegar. Take care where you wash your mild steel as the vinegar mill scale solution can stain.
Dry off. You may want to use an air blower.
Residual mill scale can be lightly removed with a grinding disc to bring up the shine.
The steel will rust if left exposed to the air so oil or MIG weld the joints straight away and then oil the parts.
Alternative - Spraying on Vinegar to Remove Mill Scale
This method is the better option for larger pieces that cannot be put into a bucket or tank but will take more patience, but is possible.
- Pour your white vinegar into an acid resistant spray bottle.
- Spray onto the mild steel, it is best if the steel can be laid horizontally as you will need to lay paper towels or use old rags or old towels onto it. Spray and soak the towels with vinegar. Leave overnight preferably for 24 hours and keep spraying with vinegar to keep the towels wet.
- Wash off with water once the mill scale is removed.
Wash away from surfaces that could be stained with the mill scale containing vinegar.
Metal Care After the Mill Scale is Removed with the Vinegar
As you now have bare metal, it will start to rust if left exposed to the air. Oil to protect from the air, leaving the joints you plan on MIG welding clean if you are going to weld immediately.
Reasons to Choose White Vinegar for Mill Scale Removal
No harsh chemicals to use.
Can be done indoors, in a garage as long as you have a lid to contain the vinegar smell.
Reasons Not To Use White Vinegar for Removing Mill Scale
It is an overnight or 24 hour process and so takes too long for some to use.
Spraying of the vinegar for larger steel pieces means there is an issue with controlling the smell of the vinegar. Spraying and leaving your project outside risks the vinegar drying out of your rags and not working or being rained on and the vinegar diluted.
This is by far the cheapest method, less caustic, requiring the least personal protection but this method of removal takes the longest. If you have the time and patience it is certainly a method to go for.
YouTube Video Demonstrating Using White Vinegar to Remove Mill Scale
Remove Mill Scale with Household Vinegar -
Video Credit: monkey fabrication garage
3. Using a Hand Grinder to Remove Mill Scale
Traditionally this has been the method to use and a MIG welders go to.
And without using the right tools there are a number of issues you could face. You may have already faced them. But before you leave in frustration hear me out.
You may have used a grinder with a normal sanding disc wheel of 24 grit zirconium. And found that the surface of your mild steel heated up. Your sanding disc clogged up and you have a scored metal surface and a shine on the mill scale.
You may have tried a grinder with a grinding wheel disc of 40 grit zirconia, gouged your thin sheet metal and yes got some of the mill scale off.
However if you want a nice surface to paint afterwards you have a problem and more grinding to do.
Flap discs and sanding disc wheels not designed for the job of removing heavy mill scale will wear out quickly, clog up and glaze over. Meaning you go through a lot of discs costing you dollars and still not remove the mill scale effectively.
The trick is to find and use a grinding wheel designed for the job of removing heavy mill scale. Here are some I've found.
A wheel that is designed to remove mill scale and leave a smooth clean surface for painting or welding on.
The disc has a diamond cutting pattern. This pattern is designed so that it helps to keep the disc cool while it is grinding. And the pattern prevents clogging.
The disc also works for heavy rust and on paint.
Galvanized steel can also be grinded without clogging the disc. This disc works well on Aluminum as it is without iron and won't embed minute pieces of iron into your Aluminum leaving it clean.
It may seem expensive but remember it comes as a pack of 25 and compared to going through many multiples of normal grinding discs I think the cost works out pretty well. And other users think so as well.
Walter Surface Technologies have a YouTube video on the FlexCut product. It is two minute 49 seconds long. Worth a look.
FlexCut Mill Scale
Video Credit: Water Surface Technologies
Another specialist grinding wheel for the removal of Mill Scale.
These wheels will also remove rust, paint and weld spatter. In the pack you get 10 discs.
These discs are well respected in the MIG welding industry as great grinding wheels for sometime. They grind without marking or gouging your mild steel.
40 Grit Pattern
I now give you something at the budget end of the market. Expect to go through a few of these when removing mill scale on mild steel. They clog up quick but they do remove the mill scale for you.
It's a good price for what it does. In each pack you get ten discs. Get a few packs if your job is large.
Metal Care after Removing Mill Scale by Grinding
The metal will rust with exposure to air but more slowly than with the acid removal methods.
Consider a light layer of oil to protect the metal from the air until you are ready to remove the oil from the steel and MIG weld or paint.
Reasons to Choose Grinding for Mill Scale Removal
No caustic acids to deal with.
With the right type of grinding wheel, it is quick and easy.
Can be done indoors or out with suitable access to power. No need to restrict your mill scale removal to the times of the year when you can safely take off mill scale using acid outside.
Reasons You May not want to choose this method
It goes without saying you need a suitable grinder and the right grinding wheels.
The right discs to use come at a cost and is more expensive than say using white vinegar.
You may have thin metal gauges and lighter mill scale to remove. If that is the case you may find the discs mentioned too aggressive and flap discs may be better for you. Have a look at a document where I outline some of the best.
You will still need suitable personal protection, goggles, face mask and gloves.
Most home hobby shop welders will have a grinder and the personal protection gear to go with it. With the right disc you can angle grind the mill scale away gently.
Allow the disc do its job of cleaning off the mill scale, without any need to lean into it. Which means just cleaning it off without gouging. If you can afford the cost of the discs this is the method I would choose.
I hope you have found my article on "Mill Scale Removal Before MIG welding Mild Steel - 3 Ways" useful and that it has given you the pro's and cons of each method.
I wish you well in your welding and easy mill scale removal using one of these methods.