Yellowish Reflection on Carburized A316L 1

[gciriani]

I need to comprehend the reason for an unwanted yellowish appearance on stainless steel, and a cure for it.

I work on a new treatment for stainless steel used for luxury accessories (high-end watch bands). The standard steel (Aisi 316L) has a nickel content (12%) which imparts the material a bright silver-white color when mirror polished. Unfortunately the treated material has a very slight yellowish appearance (invisible to the untrained eye), which is a no-no in the industry.

The treatment diffuses carbon under the surface, for 10-20 micrometers (depending on the treatment. According to the literature available, the carbon implants itself as FCC (face centered cubic lattice, i.e in the middle of the faces of each iron cube). A lab working on this asserts that the yellowish color is not superficial and appears at some depth. Would the FCC lattice explain the shift toward yellow? Is there a way of correcting this effect? Would a transparent thin coating have the effect of correcting the reflection by interference?

 

[mcguire]

Does the yellow color remain if you mirror polish the surface? If the stainless itself turns yellow, rather than the yellow coming from a thin oxide film than occurs at around 300C, then it would be from the creation of a energy gap in the 3d electrons from the addition of carbon, which would be difficult to understand.

 

[gciriani]

The yellow color remains after mirror polish. Does that mean that the 3rd electron emits slightly more energy in the wave length of yellow (aproximately 600 nm)?

 

[mcguire]

I can't think of anything else, since this is what gives metal color;i.e. electrons changing energy levels emitting a characteristic wavelength. Most metals have shared electron clouds without specific energy gaps which give a charcteristic wavelength, which is why they are all metallic grey when not oxidized.
There are experts on this topic. Prof. Michaels at CWRU in Cleveland is one. Swagelok corp also is promoting a process of this type. Try their websites and publications. Your problem is new to me, but maybe someone else has encountered
the phenomenon.

 

[SMF1964]

I would not expect the FCC lattice to be an issue, since 316 stainless is already FCC, the carbon is just diffusing into the existing lattice. If I recall my university lectures correctly, with carbon being so much smaller than iron/nickel/chromium, carbon takes up interstitial positions in the FCC lattice, not substitutional positions you describe.

At what temperature is the carbon being diffused into the steel? Stainless steel does funny things at elevated temperatures (800-1500 F) and typical carburization treatments are conducted at that temperature.

 

[mcguire]

Carbon is dissolved interstitially as is nitrogen. When it is put into the lattice at low temperatures under very high partial pressures, in this case a plasma source, it becomes
"colossally supersaturated". This is a new process about which there is little known.
You are correct that structure has nothing to do with color, but electron configuration does. Carbon acts as if it has a valence of four, I think. If we have any good solid state physicists around they could tell what the resulting 3d electron configuration is as carbon in solution is increased to high levels.