Rusting after Welding

[jimson]

G'day viewers okay I try to weld my own projects in the past after I've welded and cleaned it and then painted it I have rust come through where the weld is what am I doing wrong? jimson

[grouty]

I guess you primed it before painting.
Paints these days are not what they used to be. The tree hugger brigade have seen to most of them. Most paints these days are water based and bloody useless at stopping corrosion.
A heavy zinc bare metal primer is what is needed. Hard to get one that is any good from a local diy store. The best place is to try a steel fabrication shop that paints their RSJ's etc before they go to site.
Once you have welded it, clean it and prime it straight away. Try to avoid touching it with your hands as the moisture will start the dreaded rust ! A damp atmosphere will also start it.
I like my centraly heated workshop 

[LWC82PE]

any decent etch primer should do. i use a spies hecker etch primer that is white/light grey in colour never had any issues. .im also wondering if you are cooling the weld in water and not letting it dry off properly and some tiny droplets of water are getting traped in/ around the weld especially if its a stick weld and you have a few pits or holes and then you are painting over top of this trapped moisture? also try bead blasting the weld to get all the slag out first before painting

[Ji Gantor]

After welding you will see a blue ring around the out side of the welded section. This is oxidation which has to be removed before you paint the effected area. Grinding, sanding or as suggested sand blasting will remove the blue ring. When the oxidation has been removed clean the surface with a solvent that is recommended by the paint you are using and let dry in the sun. After the surface has dried place the frame in the shade and let the surface cool. Then without touching the surface apply your primer. Wait for the recommended time for the primmer to dry about an hour and apply the top coat.

Some people wait a week or something and the surface becomes contaminated by air bourn corrosive materials.

[Ji Gantor]

Attached is an image after welding. As can be seen there is no colour difference between the welded metal and the weld. The blue ring has be removed by light sanding.

[Ji Gantor]

To smooth out the weld I then brased the joint. Again I lightly sanded the area to remove any oxidation.

[halo1963]

What did you weld it with ?

Regards
halo1963

[Ji Gantor]

Hi Halo,
This is for information only please consult an engineer or boiler maker prior to repair.
This bracket is for holding the side number plate on. These number plates never have to come off or take any load so I used MIG. MIG welding is brittle and unsuitable for load carrying chassis members. For that I would use TIG or OXY. The extra brazing helps remove stresses from the MIG weld and reinforces the joint. But if you are welding up a load bearing member use TIG or OXY. TIG is best because it transfers less heat into the frame and there is less mess.

Ji

[lukeb1961]

MIG welding is brittle and unsuitable for load carrying chassis members.

Really? MONROE do robot MIG welds that test to 21000 lbs. (9+ tons) I'd call that a fair load.

[Ji Gantor]

Hi Lukeb,
Is that on a Chromemolly motorcycle frame.
How much will it cost for the robot to weld up a split or cracked chassis.

If this is all good please let us all know where we can get it done.

The result of 9+ tons is that shear or tension?

Most weld tests are performed slowly via a ram to gauge the strength. The test is usually singular.
Motocross chassis tubes and joints are subject to fatigue failure, that means, during their riding life they are constantly flexing. In engineering speak fatigue failure occurs after about 40 000 frequencies. This is the testing ground for motocross chassis welds. I have an engineering background of 29 years. I have been riding and fixing motocross bikes since 1975. I have seen a lot of fatigue failures and of course catastrophic failures which again in engineering speak is instantaneous failure due to crash or suspension bottoming out and the frame takes the resulting load until something gives.
I have been around Pro Supercross riders that are constantly having their new alloy chassis's welded up due to both fatigue and catastrophic failure.

A brittle weld is not what you want when you are landing the double at Conondale "Drunks Jump"   
A brittle weld can carry great loads but just try hitting a number plate bracket that has a brittle weld with a hammer to line it up. It will usually just crack off. The brittle nature of MIG welding on motocross frames may be due to not enough penetration, wrong filler rod, flux failure, contaminates, oxidation, experience of the Boiler Maker......... Most of these repairs are performed in suburban sheds or a mates workshop, neither are totally set up for this type of work. Do you know that most Boiler Makers will perform ten or twenty test welds on all new materials before moving onto the project. After all, you could become badly injured or die if the steering head breaks off while landing a jump.

MIG welding is great for all the applications it was designed to do but ask yourself, would you use it for alloy welding?


Ji

[David Lahey]

My problem is with getting paint to stick after I have done dirt bike frame welds using oxy. The grey/black coating that the paint wont stick to is very hard - resisting sanding off. It never seems to rust, but looks crap on an otherwise silver frame.
Is there an easy way to get paint to stick to that stuff?

[Ji Gantor]

Feetupfun,
I try my very best not to sand as well.
I usually place a steel wire brush into my air die grinder and remove all the contaminates.
These wire brushes sell for about $5.00 at Bunnings and last quite a while.
Any air die grinder will work as long as it has a 6mm collet.
Remember to ware protective eye ware as towards the end of the brushes life the wires fly free.
With really hard to get off stuff like baked on flux I pull out the dremel and with the aid of a grinding stone I wade through it carefully, then finish the surface off with the air wire brush.

Ji

[Ji Gantor]

Hi Feetupfun,
I have been using one of these sanding discs lately for impossible stuff and they work great. They are placed onto a 100mm grinder, then use them on steel with flakey paint just to reduce their bite. Then give it a go. They come in 60 and 80 grit. I use 80 grit and they too are available from Bunnings.

Ji

[Ji Gantor]

Sanding disc.

Ji

[halo1963]

Hi Ji Gantor
               I am a Boilermaker.

I was trying to find out if you used stainless wire becauce you may have to pasivate it.
(Heat affected zone will have carbon pick up)

Wire brushing or sanding will help however you should only use a clean stainless steel brush.

If you are using those flap disc the same will apply use a clean one otherwise you will impregnat the area with what ever you sanded last time.

TIG or MIG both will be strong enough provided you use the correct wire if you are good with a MIG you can keep the heat to a minimum.
As most people would get specific filler wire for the TIG weld they are more likely to have the right wire where they would just use what ever was in the MIG at the time.

Regards
Halo1963