Do metals that have been smelted rust, or is there a certain way to prevent them from rusting by heating them a certain way or are they stored properly. Does anyone know how it was done?
The metal most affected by corrosion would have been iron or steel. As far as we can tell, no precautions were taken to stop rust. Iron ingots were carried piled in the open in the middle of open barges:
(from Cederlund, 1998) or as ballast in ships.
How much would be lost through corrosion? Immersed in seawater, iron/steel can corrode at approximately 500 microns (0.5mm) per year. Starting with clean bare iron, initial corrosion can be a bit faster, with perhaps 1mm of corrosion before the rate drops to that 500 microns/year. How serious is this? A round iron/steel ball 10cm in diameter weighs about 4kg. If this was carried as ballast in a ship on a 1 year voyage, immersed in seawater, it might lose about 1.5mm of material, or about 350g. This is a lot, but is a worst-case scenario.
At sea, but not immersed, marine air is still a hostile environment for iron/steel. However, the corrosion rate is much slower than if immersed - approximately 100 microns/year is typical (Dillman, 2007). At this rate, our 10cm ball above would lose about 24 grams.
Away from salty sea air, corrosion rates are usually lower. Rates will depend on temperature and humidity, varying from about 5 to 50 microns per year. At a corrosion rate of 20 microns/per year, our 10cm ball would lose about 5 grams in one year.
Immersion in fresh water typically results in corrosion rates of about 5-20 microns/year (Royani, 2019), so loss would be similar to the non-marine atmospheric case above.
Many journeys would be much shorter than 1 year, and loss due to corrosion would be proportionally smaller. For a 1 week journey, the 5g/year loss would be a total loss of 0.1g.
All of the above assumes starting with clean bare iron. Iron ingots would typically be covered with black forge scale (because they are made by folding and welding the spongy bloom (which is a mix of iron and slag) from the smelter), which will reduce the corrosion rate since the black oxide is (a) stable and (b) stops oxygen from getting to the iron beneath. It would also be possible to grease ingots to reduce corrosion, but I don't know of any documentary evidence for this practice (if a reader knows of any, a reference would be great!).
References:
Cederlund, Carl Olof "Structures and vessels for transport in early Swedish iron production", Archaeonautica 14, 265-272 (1998) doi : https://doi.org/10.3406/nauti.1998.1215 https://www.persee.fr/doc/nauti_0154-1854_1998_act_14_1_1215
Dillmann, P. Beranger, G. Piccardo, P. Matthiesen, H.. (2007). Corrosion of Metallic Heritage Artefacts - Investigation, Conservation and Prediction for Long-term Behaviour, Woodhead Publishing. https://app.knovel.com/hotlink/toc/id:kpCMHAICP2/corrosion-metallic-heritage/corrosion-metallic-heritage
A Royani et al 2019, "Corrosion of carbon steel after exposure in the river of Sukabumi, West Java", IOP Conf. Ser.: Mater. Sci. Eng. 541 012031. https://doi.org/10.1088/1757-899X/541/1/012031
