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The most boring snake firework in the world lit.. and a byproduct of contact sparklers.

Dissimilar metals and a humid environment equal galvanic corrosion. An electrical potential (current) can hasten this, but is not totally necessary.

electromotive series | chemistry | Britannica.com

Your job is to determine the metals in contact here.

That will allow you to determine the potential harm, if any.

Google "galvanic corrosion".

"...equal galvanic corrosion. An electrical potential (current) can hasten this, but is not totally necessary...."

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Actually for galvanic corrosion, an electric potential (current)' is required (i.e. 'totally necessary'). This is the distinguishing freature that makes a corrosion galvanic or not.

Yes, it's a battery if all three conditions are met without any outside current flowing. My point was that the light does not have to be energized by the 120 V current flowing into the lamp for corrosion to occur.

Again, we are just adding to the confusion of the OP by going off on these tangential discussions.

OP wanted to know what the powder is and if it is harmful.

I had hoped that this."electromotive series | chemistry | Britannica.com

Your job is to determine the metals in contact here.

That will allow you to determine the potential harm, if any" would help him arrive at an answer.

The horse has been gored and will soon be rotting in the sun.

Galvanic corrosion only requires dissimilar metals and a conductive solution.

Wrong, TheProf.

The ingredients you list are necessary but insufficient. The arrangement is critical, for without proper arrangement no circuit for current is established and thus no galvanic corrosion can occur.

Note that if you drop two dissimilar metals into a conductive solution, that if the metals are not in contact, no circuit is formed, no galvanic current occurs, so no galvanic corrosion occurs.

Similarly if one metal is completely sealed within the other metal and that object is submersed in the conductive solution, no galvanic corrosions occurs because there is no current as no circuit was established.

Two containers, one of one metal, the other of another metal can both hold a conductive fluid while the outside of the containers maintain physical contact and no circuit thus no current is established, so no galvanic corrosion occurs.

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Please note that the word 'galvanic' refers to an electric current established by chemical means. Galvanic corrosion requires such a current.

Since we've probably already driven the OP off, intimate contact is not required.

"If two dissimilar metals are connected by a conducting film, an electrical potential is created between the two metals.

Electrical potential is similar to oxidation potential. If two dissimilar metals are "connected" by a solution that has ions (or free electrons) in it, an electrical potential (or difference) exists between the two metals."

Understanding Corrosion - TC-11

That is electrolytic corrosion.

Galvanic corrosion requires a current which in the case of dissimilar metals would require the metals be in contact or another source of chemically driven current be imposed.

Galvanic describes an electrical current driven by chemical means. Galvanic corrosion requires current by definition.

It's unlikely to be dangerous to touch. Just wash your hands afterward.

The guys here posted that it could be some sort of oxidation from the metal. You mention that it's on the bulb itself on the white parts. Here is a picture of one type of halogen flood light bulb - this one has white on the bulb itself. http://www.homedepot.com/p/Feit-Electric-40-Watt-Halogen-BR30-Energy-Saver-Flood-Light-Bulb-12-Pack-Q40BR30-ES-12/202216422http://www.homedepot.com/p/Feit-Electric-40-Watt-Halogen-BR30-Energy-Saver-Flood-Light-Bulb-12-Pack-Q40BR30-ES-12/202216422

If the white stuff is on the glass part of the bulb, then it's not from oxidation of the metal part. My guess is that its either some sort of mildew or some fertilizer that blew to the bulb. Either way, it's not going to harm you, unless you let it get past your skin - into a mucus membrane.

If it's on the bulb it could be residue from vaporized insects and or spider webs being their exoskeletons contain a fair amount of potassium, calcium and similar elements that when burned under a high heat will produce a whitish powder/ash residue.

So you put a bulb in your backyard about a year ago, and now there is white powder on it, and you want us to tell you what it is without a picture or knowing where you live. It sounds like dust to me. The question is what kind of dust? Do you get high when you lick the powder off the bulb? If so, it could be crack cocaine.

From what I have read, crack cocaine is produced by combining powder cocaine, water and baking soda. If it was crack cocaine, then it would be in a hard mass, not in a powder form, and you would need to smoke it to get an effect, so it's unlikely that licking crack cocaine would produce a, " high ".

This reply is really tangential.

I am really glad to see that someone pointed out that this powder could potentially cause harm if absorbed through a mucous membrane. The same could be said of most powders. A dust mask would be a sensible precaution when working with this or any dust or powder. I found I was allergic to sawdust from one particular type of tree that grows in West Africa and had to wear a mask when working with it. The dust may cause no immediate problems but if you plan to clean it off or disturb it,why take the risk. Again, the amount referred to here is very small, but still, why take a risk? Also being a floodlight I take it that the whole would be encased in a sealed casing of some sort.

I just wanted to say I've encountered this harmless white dust with halogens hundreds of times in the past... (In the past.) What in the world would you be doing with a new halogen outside fixture when an LED alternative is available?

LED bulbs use less electricity than halogen bulbs. LED bulbs typically outlast halogen bulbs. LED bulbs generate less heat than halogen bulbs. Hmmm sounds like LED is the winner, except ... If the light is on an old style dimmer, you probably can't use an LED bulb.

<...Is it harmful...> Everything has the potential to do harm.

If it's o the white plastic parts, it's most likely plastic that has been UV damaged. Between the heat and the UV damage, the plastic takes a beating. Again, probably relatively harmless.

In summary though,

Don't lick it.

If you must lick it, turn off the light and let it cool first.

If you must lick it and can't wait for it to cool, don't bother telling us about it.

If you must lick it, can't wait for it to cool and must tell us about it, expect at least one, "I told you so".

I don't think there's plastic on Halogen bulbs, due to the heat generated. Since it's an outdoor light, I'm thinking its a PAR38 or maybe PAR30, it should be all glass (part that's exposed to the elements). Maybe the aluminum thread may show a tiny bit, but no plastic.

If the ends are ceramic, it's breakdown of the ceramic due to heat cycling.

Repeat cautions:

Don't lick it

If you must lick it, turn off the light first and let it cool.

And so on......