An Anod(yn)e Dock

This week my in-laws, Scott and Athena, bought a new dock for their lake. They decided on one made from anodized aluminum, and we were talking about its advantages over other materials. They told me that it doesn't heat up as much as other materials, and this made sense to me from baking with aluminum pans: They tend to heat up quickly and evenly, but there's also very little risk of burning yourself on them. I thought this was due to the heat capacity, the relationship between heat and temperature. Colloquially, we tend to equate these, but there's an important difference: Heat measures the internal energy of a substance, while temperature tells how easily it will give up that energy. Heat flows from high temperature to low temperature. Heat capacity measures how quickly temperature changes as heat flows in or out – Water has a relatively high heat capacity, which is why even a small amount of hot water can burn you. Previously, I had thought aluminum's low heat capacity meant that if you touched a high-temperature pan, your finger would cool it much faster than it would heat your finger. It turns out there's a bit more to it than that.

Scott and Athena mentioned that it was important their dock was made from anodized aluminum, rather than natural aluminum. Anodizing is a process that adds a layer of oxide to the surface of a metal, protecting it from corrosion. In the dock's case though, they said that this also made it feel much cooler than the natural aluminum, which would get uncomfortably hot in the sun. This didn't work with my explanation, since anodizing is a surface effect, which wouldn't significantly change the heat capacity of the bulk material. I decided to compare both the heat capacity and the thermal conductivity, which measures how quickly heat flows through substances:

Material	Heat Cap. [J/kg K]	Thermal Cond. [W/m K]
Stainless Steel	502	14.4
Natural Aluminum	921	236
Anodized Aluminum	921	1.07

Contrary to my previous understanding, aluminum actually has a higher heat capacity than steel! In reality the key difference is in the thermal conductivity, for which the three metals have vastly different values.