Rust in the aquarium / Passivating stainless steel

OK, so I owe the titanium fanboys an apology. In my original post about aquarium chillers, I asserted that a stainless steel chiller coil would not rust in a saltwater aquarium. The coil in my tank rusted after about 6 months. Take a look:

I decided to sandblast the part so that I could see how extensive the damage was.

Yeow! The metal must have been exceptionally thin, and the sandblasting blew right through it!

I used 316 SS filler rod, and TIG welded the hole shut. The blue tube is an argon hose to protect the inside of the stainless tube from oxidation while welding.


Now it's all patched up. The heavy corrosion was localized around the area where the stainless coil entered the plastic filter box (see top picture) . My guess is that vibration from the pump caused the rough surface of the plastic to abrade the passivated layer on the stainless, thus causing a localized spot where the metal was unprotected. I've heard that passivated the stainless in an acid bath can create a stronger, more uniform passive layer. There are two common acids used to passivate stainless: nitric and citric. Nitric acid is nasty stuff, and it's possible to damage the stainless parts if the procedure is done incorrectly. Citric acid seems to be just as effective as nitric, and it's non-toxic. Here is the best reference on citric acid passivation:

http://www.astropak.com/downloads/technical_papers/boeing_passivation.pdf

I mixed a %15-by-weight solution of citric acid in water. I got 2lbs of citric acid on eBay for under $10 shipped. I submerged the stainless coil in the solution for 2 hours at room temperature. I could see the surface changed a bit -- the shade of gray was a little different. Hopefully this means it built up a nice strong oxide layer.

I've put the coil back into the aquarium, and added some silicone pads to the filter box so that the plastic would not scrape away at the stainless coil. I'll keep you updated to see how effective it is.

Monster TIG nozzle = monster waste of money

UPDATE 6/15/2013: Arc-zone has redesigned their Monster TIG nozzle. It now has a chunk of rock wool or similar material to help diffuse the gas flow. I have never used this version, and so the comments in this blog post do not apply to it. Check the comments section for more details.


Today's lesson involves my quest to weld stainless steel sheet metal, a Monster TIG nozzle, and a copper chill bar. In previous posts, I've described having trouble maintaining weld bead quality on thin stainless sheets. The problems are a combination of putting too much heat into the metal, and having too little argon gas coverage. I am not sure if addressing one problem can help solve the other. Today, I did some testing to find out if adding a lot of gas coverage can help. I also tested out a copper chill bar.

Common weld parameters for the whole test:
1/16 ceriated tungsten ground to a sharp point
55 amps (pedal floored for the entire test)
very slight %90 pulse at 200Hz just to get my auto-darkening helmet to work
20 CFH pure argon
10 sec post-flow

I purposefully used a small piece of 304 1/16" sheet to show the heat buildup problems. I also welded close to the edge to test the worst-case heat buildup.

First up: normal gas lens with #8 cup.

Wow, I never knew the copper could help that much!

Next, a large diameter gas lens with #12 cup
Same story here. It looks like there was even less heat in the metal. This might be because there was better contact between the sheet and copper, or because the gas nozzle has a wider opening. I'll bet the Monster nozzle will be even better...

Finally, the "Monster TIG nozzle", which is 1" in diameter and uses a stubby gas lens collet body.
Wha?! There must be something wrong -- what's going on here?! I tried all gas flow settings from 5 CFH up to 30 CFH and concluded this nozzle is completely useless. It's possible that I am misunderstanding something since I am a new welder, but I am pretty sure this thing just plain doesn't work. I noticed that the tungsten had turned black after a few welds, indicating the gas coverage isn't even enough to keep the tunsten from oxidizing. At 30 CFH, the gas flow was so turbulent, I could see the arc getting blown around, and pops of smoke coming out of the weld. At lower flow settings, I could see the stainless oxidizing even before I lifted my hood. I tried different stickout from 1/8" up to 3/4" with no change. I am sure the cup made good contact with the torch body, and there were no air leaks. I even tried extending the nozzle away from the gas lens with a spacer to make sure there was adquate space for the gas to disperse with only a tiny improvement.

On the right: large gas lens with #12 cup. On the left, you guessed it, Monster suck.



This screen arrangement doesn't look so great.

It's made with just two screens without any spacers between them, and two very coarse screens on the outsides. The screen diameter is a few mm less than the interior diameter of the ceramic cup, so I'm guessing a lot of gas slips around the edges of the screens.
So, I'll be continuing my stainless welding quest without the Monster nozzle and with copper chill blocks. I'll also be testing Solar Flux B. So far, I think it works well but poses a huge cleanup mess after the welding is complete.

Improved stainless welds with large gas lens

My new TIG parts just arrived today, and I had to do a quick test. In short, the large gas lens makes a huge difference. Take a look at this beast: #12 cup with "large gas lens" and 1/16 tungsten


I am using the same exact 1/16" thick SS304 sheet metal that I have been practicing with all along:

Compare that weld bead to the pair of beads in my previous post. All settings are exactly the same except for the gas lens. Well, I guess I was using .040" tungsten in the previous post, but that should have helped if anything.


Here's a few more. I've heard that "salmon color" is the best thing a stainless welder can hope for. I changed the flow rate from 10 to 20 CFH going from top to bottom -- not much difference. The bottom bead had a longer post-flow, so the tail of the bead has less purple/blue.

The backsides of these welds are pretty messed up. I'll report about the Solar flux later.

First attemps at TIG welding stainless steel

I've spent most of my TIG welding practice time on aluminum, but have recently started to experiment with stainless steel (all 304 for now). It's difficult! Most welders say that aluminum is the most difficult because it liquefies quickly and has oxide layer problems, but in my opinion, stainless is more difficult because of the shield gas requirements.
Here's my setup:
.040" tungsten -- my local welding shop convinced me to try this instead of 1/16" tungsten. They somehow thought I could weld with less heat while using .040". I didn't understand it when they explained it so me, and I still don't -- especially since it makes no difference that I can tell.

no. 8 cup with gas lens

less than 1/4" stickout




Here's a 1/16" thick 304 sheet. The upper bead was done with 10CFH argon, the lower bead was 20 CFH. The picture shows the front and back of a simple bead with 1/16" filler. I was using as absolutely little heat as possible, sometimes solidifying the weld pool as I moved, making for an irregular bead. These beads both have major problems. They are pretty gray except on the left side where I finshed the weld and the post-flow cooled off the bead. The rest of the weld cooled outside of the gas shield, turning it gray, which is bad because the structure of the stainless steel has been altered. This will likely lead to corrosion or stress cracks.

I prepped the left side of the top surface to see if it would be any different than the un-prepped right side. The metal has a smooth almost plastic-like feel, so I was unsure if this was normal. It didn't seem to make any difference.

The backside shows major problems too. The upper bead looks better because there was more sheet metal all around the weld to soak up excess heat. The lower bead is "sugared" because it got very hot in an oxygen atmosphere. The weld is close to the edge of the sheet metal, so the heat built up more quickly.

To attempt to fix the topside problems, I've ordered a "large gas lens" setup with some huge gas cups from an online welding store. I tried to order these parts at my local welding shop, but they didn't have them, and were somewhat hesitant to even order them for me. I have no idea if the large gas lens will work, but the parts are relatively cheap.

To attempt to fix the backside problems, I've ordered some Solar Flux B, which was originally made for gas welding stainless steel. The alternative to flux is to build an argon purge for the backside of welds. This purge could be a box, a nozzle, or some other device to make sure the backside is flooded with argon. The problem is that every weld situation (tubes, sheet metal, angle, etc, etc) requires a custom purge setup, and argon isn't exactly cheap either. I'll definitely be posting more information about the flux in the coming weeks.


Here's a weld made on thicker (about 1/8") stainless. The grade of stainless is unknown. Don't mind the soot. I was just too lazy to regrind my tungsten after I hit it with the filler rod. Notice the bead is NOT gray. The thick metal is able to pull heat out quickly enough to prevent problems. Unfortunately, most of my stainless welding will be on tanks and tubes that will never be 1/8" thick. I need to find a solution that will work down to .049" at least.