buffing out the scratches

[Ted R.]

I am considering purchasing a used brite tank that has some green scrubby scratches on the interior. What should be used to polish the scratches out and give it that shinny new look and clean-ability?

Thanks.

[Woolsocks]

All I can think of is to get a welding-type-person to buff it as if they had just freshly welded it. Someone else may have a better idea.

[NESTAFL]

I have used a polishing wheel on the end of drill to polish out some rust specs. It worked well but there was an obvious patch that had been polished. I followed with a passivation.

Jordan

[gabewilson50]

A hand grinder fitted with a polishing wheel (120 grit) works quite nicely, or you can get some acid and some of the red scotch brite pads (specifically designed for cleaning stainless steel) and hand do it, or a combo of both. That's what my stainless welder did for anything that needed cleaning or polishing. Acid should be a nice strong inorganic acid (HCl aka muriatic acid or HF in gel form).

[gitchegumee]

Unless your tank interior is mirror finish--and I haven't seen many of these--chances are that your scratches are NOT from a green 3M "Scotch-Brite" pad. Tank manufacturers use these pads as a first choice when buffing and are WAY less abrasive than 120 grit. I've said it before, and I'll say it again: Scotch-Brite is safe for tank interiors. I would suggest using 180-220 grit to get the scratches out, then buffing with "Scotch-Brite". Follow that with passivation using 50% nitric acid at around 30C for 45 minutes. Open the tank and let it air dry. Good luck!

[Moonlight]

Try to only use the scrubbies in the same direction instead of circles or random actions. Ideally use a repeatable pattern- like around the tank, not up and down. This alone will solve your problem. If you have ever sanded wood, it is just like sanding with the grain.

[gabewilson50]

Follow that with passivation using 50% nitric acid at around 30C for 45 minutes. Open the tank and let it air dry. Good luck!
I also had always heard (from other brewers) that we want the tank to air dry after acid treatment to passivate. I have a crapload of SS surfaces that I'm going to want to passivate before I start them into service, so I started looking up how to passivate, and all of the metalworking sites I visited were adamant about rinsing the acid off immediately following treatment with cold water and not letting any of the acid dry before the rinse. I haven't sprung for the ASTM A 380 document that spells it out, but everything else mentions rinsing. Are we incorrectly passivating (or worse, not really passivating) our steel? Does anyone have any input on this? Any help would be great.

Thanks,

--Gabe

[NESTAFL]

I used to hear the theory of air dry after acid passivation and even did it a couple of times. About year and half ago, I was setting up a new place so I talked a lot with the chemical supplier. He kinda chuckled at the air dry thing and said with confidence "rinse". So i did. All I can say is everything has been good and I feel he would know best.

[Kevin O'Connell]

The guys that recomended scotchbrite are on the right track. Scotchbrite makes various grit abrasive pads and backing plates that will mount on die grinders or drills. Die grinders are optimum due to the higher RPMs they turn. Be sure not to over rev them in accordance with RPM limits listed on them.
For mirror polishing, use a buffing wheel and Jeweller's rouge. It also comes in various grits to make it cut faster. As stated in other post, acidizing after using the rouge will ensure the residues are out of the tank.
You can call me at 620-875-1083 to discuss further. Kevin.

[gitchegumee]

After further investigation, I am apparently wrong in recommending an air dry after passivation. According to ASTM A380-06, section A2.10:

"... To minimize staining, surfaces must not be permitted to dry between successive steps of the acid cleaning or passivation and rinsing procedure. Thorough drying should follow the final water rinse."

NESTAFL and Gabe were right on the money. Sorry for wrong information. Guess I was part of the misinformation campaign of "old brewers' tales" that NESTAFL cited. Furthermore, this ASTM document applies only to new equipment, not used. Don't know how it would affect anything, but the paper did specifically state that it covers new equipment only. It goes on in section 1.2:

"This practice does not cover decontamination or cleaning of equipment or systems that have been in service....... On the other hand, some of the practices may be applicable for these purposes."

No reference was given for any other method that may apply to used equipment. Always pays to do your own due diligence and not rely solely on what's posted here. Again, my apologies for the wrong information.

[chaser]

I also had always thought you were supposed to air dry after passivation until some of the guys from Mueller were at the brewery a year or so ago to work on some of our tanks. They kind of freaked out when we mentioned air drying after passivation and explained why you MUST rinse after passivating. Good stuff to read the morning I am about to passivate a new brewhouse.

[gitchegumee]

I've seen what nitric/hydrofluoric acid pickling paste will do to stainless at room temps in just a few minutes. Don't want my mirror polished tanks to etch to a frosted finish! I have 21 tanks to passivate and can't do it all in a day. Anyone leave warm nitric in a tank over night? Should I be concerned?

[dick murton]

Have to say that I have never passivated new, let alone secondhand tanks before. New tanks should all be passivated before they leave the manufacturers. Make sure your contract states that. The only places that get pickled once on a production site are the welds where they have been exposed to air, to ensure that the blued area is thoroughly oxidised, to reform the protective chromium dioxide layer. What I mean by that is that passivation should not be necessary on the inside of pipes etc providing the pipe has been thoroughly purged with Argon. However, the outside does not normally get an all over gas purge, and turns blue, and it is this blued area in particular that needs passivating. Hence the use of nitric acid. I assume that hydrofluoric acid (in lickling paste / gel) also oxidises, but haven't a clue about the formula for this stuff. All I know is that it is nasty stuff to get on skin or clothes.

Nitric acid is about the best relatively weak (compared to hydrofluoric) acid to leave in tanks, though once the new welds have been pickled, I really don't see why you need to use it. Make sure it is completely clean (degrease) first. If there is any grease left on the surface, then the acid will not cut throught it, and will not passivate if necessary. 3 - 5% caustic @ 80 - 85 deg C for 30 minutes should degrease OK first.