Cleaning mild steel after welding first created 22/12/08 - last modified 22/12/08 Page Author: Ty Harness
Warning

Do NOT Drink or inhale the fumes produced by the following products and follow the manufacturer's safety guidelines. Keep it off your skin and out of your eyes by using protective safety wear.

Gas welding leaves behind a great deal of scale compared to TIG, MIG and MMA. TIG welding is the cleanest process with virtually no scale of spatter produced. Figure 1 shows some typical welding scale left behind by the gas welding process. The scale is difficult to remove even by wire brushing. There's no excuse for the rusting on a real job but this transformer was pulled out of the skip and will serve as a cleaning example for this tutorial.



text here
Figure 1 - Sheet metal transformer covered in welding scale and rust. Partially submerged in diluted hydrochloric acid


To remove the oxidation layers and scale from the welding process then you can use a wire brush preferably on a drill or pedestal grinder or fine emery abrasive but be careful not to remove material or leave grind marks. Alternatively then an acid solution in either a gel paste or liquid form can be used to pickle the scale off. Diluted hydrochloric acid [1] is a good remover of oxidation and scale. Always add the acid to the water and following the manufacturer's instructions.

From a DIY point of view, I find brick and patio cleaners are good in that they are already very weak solutions of hydrochloric acid and they do NOT contravene the water bye-laws and hence you do not need a license to use them. The instructions will guide you further with dilution with water. For example the Cementone(TM) product [2]: 'Brick and Patio Cleaner' is 9.9% Hydrochloric acid and the instructions say you can use it neat to clean rust or mortar from tools but I find it's far to strong and attacks the metal. I dilute a further 1 part Brick and Patio Cleaner to 5 parts water and leave for 2 hours fully submerged. I find this is sufficient to loosen the oxidation and scale. Larger jobs may need partial submerging so I keep turning the work every hour. Figure 2 shows the transformer removed from the acid after 1 hour.

text here
Figure 2 - Mild steel cleaned using hydrochloric acid




Clean well with water to remove the acid.

Repeat process if needed.

You need to get metal protector, or fine coating of oil on very quickly before rusting sets in.

De greasing will be needed if your going to (say) paint the work piece at a later stage.



text here text here
Figure 3a, b - Using Deoxgel[3]


Figure 3a shows I've left a quarter of the transformer uncleaned with the aim of testing a new product for this tutorial called Deoxgel(TM) [3] which is a gel based rust remover. The benefit of a gel is that you don't need to dip or turn the work piece in liquid so it can be used on vertical or overhead surfaces. Spread it on thick leave as long as required and then scrape and finally wash off with water. It takes rust off equally as good as the acid but I find it requires a 24 hour application to remove the welding scale. Figure 3b shows the gel after 24 hours and figure 4 shows the transformer washed and wire brushed ready for painting.

text here
Figure 4 - Mild steel cleaned and ready for painting.




There's little benefit to polishing mild steel to a shine because it soon turns dull and a protective coating such as paint needs to be applied to prevent rusting.
References

[1] http://en.wikipedia.org/wiki/Hydrochloric_acid
[2] http://www.bostik.co.uk/diy/product/139
[3] http://www.bilthamber.com/deoxgel.html




Tested in Firefox 3.05: FireFox homepage