Sandblasting Cast Iron - This is really Weird...

HotRodHudson · Apr 30, 2018

I have media blasted a couple different cast iron items over the last week and have noticed something weird. With in 10-12 hours of completion, there will be areas which were blasted clean that now show up as "missed areas" and they are darkened back up to a dark rust color.

These areas are not a solid spot, it could be an area the size of a pea but within that area its not solid discolored but more variated in color. Other times there is a bit of what I would call veining, or even just some of the raised ridge areas of the material. I have used both glass bead 80 mesh alone and alum oxide 80 mesh alone on different pieces with similar odd issue developing later on each item.

I will try to include a photo or two to show what I am talking about but I dont understand why this is showing up later, hours later?

ddawg16 · Apr 30, 2018

I've sandblasted CI before....but the only issue I noted was how fast it would rust....

Cast Iron is real porous....is it possible you had some oil in there and it wicked up?

HotRodHudson · Apr 30, 2018

Here are a couple photos of items which were all blasted uniformly when done.

The exhaust manifold was done a couple days ago but it’s spreading. Yesterday morning this discoloration wasn’t present, it was just a few specks now it’s like a growing rash!

Sent from my iPhone using The Garage Journal mobile app

metlmunchr · Apr 30, 2018

Wild guess would be oil migrating to the surface.

Lelandwelds · Apr 30, 2018

The flakes, spheres, and veins of carbon that are rejected from solution as the iron cools to a solid make cast iron a unique material.

iajonesy · Apr 30, 2018

Not 100% sure if it's oil or rust starting again, but I would try heating the piece as long as it isn't structural, to draw out any oil that might be in the pores. I would blast it again after heating. As stated above CI is like a sponge and oil will wick into the metal over time and it will rust in a heart beat after blasting.
If 'Cuda chick sees this post she might have some good advice for you.

Mike

ddawg16 · Apr 30, 2018

Considering the location....I'm going to say it's oil migrating up. That is the area that typically sees oil when you have a valve cover leak

Nor'Easter · May 1, 2018

It's called flash rust. Seen it hundreds of times. Paint it sooner if you're worried about it.

wimpy525 · May 1, 2018

Nor'Easter said:
It's called flash rust. Seen it hundreds of times. Paint it sooner if you're worried about it.

This is correct. I sell shot blast systems, I see this happen with in a matter of hours in certain locations. I have had new technicians pick up parts with bare hands and an hour or two later rusted finger prints on the part.
I was also part of a team that was testing a device that would measure profile on a plate of blasted steel as it left the machine. We tested the part and recorded the findings, went to lunch when we returned we tested the plate again and readings where different. We started trouble shooting the device only to find nothing wrong. Turns out the plate had flash rusted and was not visible to the eye.

WoodsTruck · May 1, 2018

I had the front drum brakes of my Bronco on the bench and wanted to clean and paint them. I didn't blast them, but got them pretty clean with a wire wheel. Out of curiosity, I grabbed the propane torch and drove the water out of the drums, then painted with high temp rattle can paint. I was amazed how much water would run off the drum as I heated it. Your manifolds are likely no different.

ddawg16 · May 1, 2018

I've had freshly blasted parts rust quickly....but haven't heard the term 'Flash Rust'. So I looked it up....

http://flashrust.org/2014/03/06/defining-rust-back-flash-rust-and-rust-bloom/

The key elements in categorizing rust as flash rust are as follows.

• The rust appears after a wet method of surface preparation

• The rust develops within minutes or hours

• The rust is somewhat uniform in color and texture over large areas

• The rust is tightly adherent if it is light, and powdery if it is heavy

• The rust is mostly the result of normal oxidation and not accelerated by chemical contamination

• The rust is usually evenly dispersed

I don't think the OP's issue meets the criteria of flash rust. Something else is going on.

If it was flash rust, I would expect it to be all over the manifold.

Give the location of the 'rust', I still think it's oil wicking up from the pores of the cast iron. That location of the manifold typically sees oil leaks from the valve cover. And because it's next to the head, does not see as hot of temps as the rest of the manifold, hence, the oil is not cooked off.

Lonnies Performance · May 1, 2018

WoodsTruck said:
I had the front drum brakes of my Bronco on the bench and wanted to clean and paint them. I didn't blast them, but got them pretty clean with a wire wheel. Out of curiosity, I grabbed the propane torch and drove the water out of the drums, then painted with high temp rattle can paint. I was amazed how much water would run off the drum as I heated it. Your manifolds are likely no different.

The water you are seeing is the byproduct of burning propane, not the moisture in the metal.

gofastwclass · May 1, 2018

Lonnies Performance said:
The water you are seeing is the byproduct of burning propane, not the moisture in the metal.

Agreed. We talked about this when I was in school.

CudaChick1968 · May 3, 2018

Thanks for the mention @iajonsey and for reaching out HotRodHudson! I've experienced the same thing here mainly on cast iron intake manifolds when I run out of steam at the end of a day and leave it overnight in my blast cabinet -- some areas look like they were never blasted and others don't change to the naked eye. I've always just figured it was flash rust for lack of a better term because a fresh blast gets rid of it long enough for me to get it in powder.

Every metal tries to protect or "defend" itself in some way which we view as corrosion. Steel rusts, aluminum oxidizes, magnesium turns black, etc. If you left your pieces alone for awhile my bet is that the whole thing will look like your close ups eventually. I'd be willing to bet it's just certain metals within the cast iron corroding at a different rate.

You didn't really say what you're prepping the cast iron for so my powder prep (which varies greatly from paint) may or may not be any help to you but I'm happy to share what I can. Just give me a call any time after 10 a.m.

Willie Makeit · May 4, 2018

ddawg16 said:
I've had freshly blasted parts rust quickly....but haven't heard the term 'Flash Rust'. So I looked it up....

http://flashrust.org/2014/03/06/defining-rust-back-flash-rust-and-rust-bloom/

I don't think the OP's issue meets the criteria of flash rust. Something else is going on.

If it was flash rust, I would expect it to be all over the manifold.

Give the location of the 'rust', I still think it's oil wicking up from the pores of the cast iron. That location of the manifold typically sees oil leaks from the valve cover. And because it's next to the head, does not see as hot of temps as the rest of the manifold, hence, the oil is not cooked off.

so, by your own admittance you don't know what flash rust is but you know it isn't the OP's problem. :spit:

HotRodHudson · May 4, 2018

Thank you all for your thoughts on my issue. After Blasting the exhaust manifold the first day, we then had rain for a day or two. So humidity was high 85% or more, and our normal is below 30% as an average.

Since I tore apart my blast cabinet to make some needed mods, I haven't tried re-blasting the manifold again, however it hasn't gotten much worse than the photos I shared originally. I would think it's either a rust issue or perhaps the make up at those points is a higher content and rusts more readily. I don't think it would be oil as the exhaust manifold is on top of the engine on the old Hudson and I had it running quite a few times to burn anything off before disassembly. However, I could be wrong and oil or contaminants could be there.

I will give it a good chemical cleaning (lacquer thinner) prior to re-blasting it again just to be sure. Then I will be sure to prime and paint it immediately after blasting just to be safe.

drivesitfar · May 4, 2018

HRH: I don't know what the OFFICIAL TERMINOLOGY is, but I've been calling it FLASH RUSTING if I wire wheel off my cast iron down to bare metal and don't do anything to it then the FLASH RUST or rust color starts almost immediately here in the PNW. I usually try to put on BLO (boiled linseed oil right away) to prevent the rusting even if i'm planning on painting it, but I like naked steel look. i've heard that taking cast iron out of E tanks will flash rust after cleaning so guessing sandblasting maybe has the same issue.

just curious if sticking the cleaned part in some sort of air tight area until you are finished with it might help if you don't want to put BLO, wax or some other rust preventative on it?

good luck with your project.

Cuda: thanks for the 411 you posted!!

BillK · May 5, 2018

hhh,
We see that a lot on marine engines that have been in a salt water environment. Back when I first started in business I milled a cylinder head right at the end of the day and left it sitting on the machine. The next day I came in and it looked like I had never touched it

Now anything marine gets sprayed with WD40 as soon as I finish machining it.

We even see it after oven baking blocks and heads at 500 degrees and then shot blasting them.

I would guess that yours is from road salt getting into the metal over the years.

Kevin54 · May 5, 2018

Ditto on the flash rust. Once things are blasted, you open up some pores on the cast iron. Cast Iron is one of the quickest materials to rust. Use CLR or Evap-Orust, and be done with it.

K13 · May 9, 2018

I agree flash rust especially considering the humidity you experienced and touching it with your bare hands is not good either as the moisture and salt from the sweat from your hands can speed it up in the areas you touch.

Sandblasting Cast Iron - This is really Weird...

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