How hot can metal get in sunlight? I will make some 5mm blackened steel letters for a building. Now, in the sunlight in summer, how hot can they get?
EDIT:
i want to place some LEDs on the back of the letters. now the question is if i have to watch out for high temperatures or if i have to use a special glue. There are some people who say that the letters can heat up to 90°C in Summer while others are saying that the Letters will only heat up to the air temperature or a little bit higher.
So, im in europe, not in the desert but i just wanted to clarify who is nearer to the answer.
90°C or about 30°C ... Depending on the answer i will attach the lighting.
 A: While the answer for your particular case depends on a lot of factors, you can estimate how hot an object gets that is facing the sun (and whose back side is insulated) using the Stefan-Boltzmann law. Let us assume that the material is perfectly black (emissivity of 1.0) and facing the sun which delivers approximately 1 kW of power per square meter at the surface of the Earth (a bit more at the top of the atmosphere).
The black surface will heat up until it loses that kW as fast as it is coming in. It will do so by a combination of radiation and convection.
We all know that things inside a car can get very hot - this is because the windows reflect more radiation at long wavelengths (the "greenhouse" effect). But for this calculation I will initially ignore this effect although it does play a role.
Stefan-Boltzmann: $$J = \sigma T^4$$
To lose 1 kW over 1 m2 requires a temperature of $$T=\sqrt[4]{\frac{1000}{5.67\cdot 10^{-8}}}\approx 364\ \mathrm{K}$$
This assumes only the surface facing the sun loses heat by radiation: in other words this is only valid for a black surface mounted on a good insulator. It is an upper limit when we ignore convection and greenhouse effect (which are opposite - convection will cause the material to be cooler, and greenhouse effect will make it hotter). It is clearly not enough to melt your lettering.
Incidentally there are some interesting links on the temperature of objects in the sun. I quite liked this one[archived] describing measurements on cars. It shows that things left in the sun can get significantly hotter than the surrounding air (although there are some issues with the method used, the conclusions are mostly valid),
One final note - how hot things are is not the same as how hot they seem to be. Touching a good conductor (metal) will exaggerate the apparent temperature (hot feels hotter and cold feels colder) when compared to touching thermally insulating materials; and using non contact thermometers can lead to errors when comparing surfaces of different colors (emissivity) - but not enough to explain the reading of 160 °F mentioned in the above article.
A: If the building is located in Hammerfest, Norway the metal might get warm to the touch. If it's on the sun-facing side in Las Vegas you may find it gets hot enough to cause burns. A definitive answer is not possible.
If people can touch it easily the temperature is a concern. If it's out of reach, don't worry about it. The iron isn't going to melt.
"blackened steel" isn't very descriptive. What kind of steel, and what kind of coating? Mild steel with a chemical treatment will rust out in months. Faster if the place is on the ocean. Stainless is hard to treat. Paint works well (provided it's the right paint), and powder coating is particularly durable. Best to call an outdoor sign shop in the installation's area and ask them what works best in that climate.
Aluminum is very popular for outdoor signage, consider changing materials if you haven't finished already. Anodized aluminum is very durable and available in a wide selection of colours. Watch the fasteners though - you can't use steel screws (not even stainless ones) in aluminum plate.
