We provide detailed insight into the growth kinetics of intermetallic layers formed at the interface of molten Al-12%Si with cylindrical encapsulations of 316L stainless steel. This knowledge is relevant for the design and operation of high-temperature latent heat storage and energy harvesting devices at application-relevant conditions. Our continuous experiments on iron-aluminide deposition rates at 600 degrees C and 700 degrees C lasted up to 120 days. The layer thickness was determined in metallographic cross sections by light microscopy. We observed deviations from standard growth models, including a delayed layer growth onset at 600 degrees C and strongly nonparabolic kinetics at 700 degrees C. We developed a numerical diffusion model that accommodated temperature dependent kinetics and explained the observed deviations by incorporating de-passivation and dissolution phenomena. Numerical fitting of the model to the experiments provided optimized mobility parameters that agree with those reported for the Fe2Al5 compound, whose presence was confirmed by energy dispersive X-ray elemental analysis. Differential scanning calorimetry revealed reductions in heat of fusion and melting temperature, of -10% and -10 K, respectively, after 120 days at 700 degrees C. Fe concentration in the melt increased from < 0.3% to 2.6%, under the same conditions. The validated model was then used to provide design guidance for heat storage applications including degradation. For 8760 h of full-load-charging: a energy density of 370 kJ/kg was conserved, with variable source temperature distribution. For high power density configurations (1430 kW/m(3)) narrow source temperature distributions were beneficial. Our multi-month experimental campaigns enable more accurate degradation and performance predictions and provide guidelines that result in better long-term performance of the Al-12%Si phase change media.