Mild steel chemical reactions

[metalurge]

Does not appear that my attachment attached... I am trying again:

I have treated sheets of 16 gauge mild steel to produce results as attached in "Steel 1". What I used to achieve this result was to first wash the steel in Phosphoric acid. Leave for 3 hours, then wash with the blacking solution found at this page:

http://www.metalfinishesplus.com/Qmidnight.html

- I am not sure what is in this... selenium dioxide maybe?

Later I have tried to reproduce these effects but have not been successful. Following the exact same steps I have only been able to produce the effect shown in "Steel 2". Any ideas what may be going on? The only thing that changed was the steel... I am using the same dealer but its not the same batch of steel- the first batch I bought months ago, the second recently. I can't seem to understand how that would make such a big difference. What I am most concerned about is the loss of the reds and blues... Any ideas out there.

 

[unclesyd]

What type finish are you looking for, uniformed, mottled, or other?

First off your sample #1 doesnt appear to have a uniform finish while the #2 the finish apparently hasn't taken. If #1 was caused by contaminates it will extremely hard to duplicate.
I would start looking at the cleanliness of the starting plate. Steel sheet can come with all manner of coatings to prevent rust, etc. I notice that it is recommended that the objects be grit blasted, which would be after a chemical cleaning with something other than H3PO4. While H3PO4 is a good deoxidizer and metal prep it sometimes fails to clean oils off metals.
I would run a few tests with chemically cleaning, grit blasting, or H3PO4 treatment or any combination of the above.

One other point is that while the brochure says no known shelf life this normally means for the material in sealed container not in a bath or one that has been exposed to air.

 

[metalurge]

Thanks unclesyd,

The first sheet or "metal 1" is exactly what I am shooting for... this kind of un-uniform splotchy look with lots of reds and blues which incidentally is NOT what the intended purpose of that chemical is for. I found that although the chemical didn't do what it was advertised or supposed to do, the effect I got was perfect for me.

all the sheets that I am working with are etched with acid first- so I could be mistaken but I felt pretty confident that both sheets 1 and 2 probably had the same "cleanliness" to them when it was time for me to apply the second chemical. Its when i apply that second chemical that things change... Its very difficult to produce those vibrant colors...

 

[unclesyd]

Splotchy blacking is as stated above usually due to cleanliness of the metal surface.
In the OP you stated that you wash with H3PO4 then leave for 3 hours.
Do you leave the metal in the acid for three hours or do you etch and leave the metal exposed to air for three hours?

What is your acid strength and temperature?


#1 may have been achieved due to flash oxidation or rust on the sheet. It appears that your blacking process is looking for an active surface. Different times of the year would give different results based on the humidity.

 

[metalurge]

hmmm... ok, I put the acid on and left it for 3 hours.... then washed it off and immediately put the blacking on for just a few moments/minutes. What is flash oxidation actually?

 

[unclesyd]

Flash oxidation or flash rust immediately begins to form when you expose an active steel surface to the atmosphere. How fast it forms depends on the temperature and humidity.
There hours is a very long time to leave steel in an acid bath.