The work presented in this thesis focuses on the effect of a temperature gradient on the local composition and morphology of ash deposits formed on heat exchanger tubes of power boilers. Cross-sections of superheater deposits from a Brazilian and Finnish kraft recovery boiler were analysed using Scanning Electron Microscopy/Energy-Dispersive X-ray Analysis (SEM/EDXA). The data obtained from the Finnish superheater deposits was complemented by deposition probe measurements, that were carried out in the superheater region of the same boiler. In addition, laboratory-scale experiments were carried out using synthetic ash deposits to study the initial melting behaviour of deposits and, by varying the deposit composition, whether the identified deposit ageing mechanisms are also applicable to deposits other than those found in kraft recovery boilers. In the Brazilian superheater deposits, all three deposit ageing mechanisms identified in previously reported laboratory studies (i.e. temperature gradient zone melting, movement of enriched melt toward the steel, and diffusional transport of alkali chloride vapours toward the steel) were confirmed to also occur in actual superheater deposits. In addition, the movement of a melt and concurrent enrichment in K toward the steel, induced by a temperature gradient, was identified. Due to the local K enrichment in the vicinity of the superheater steel surface, the local first melting temperature of the deposit decreased significantly and reached a minimum where the local K concentration was the highest. The ageing behaviour of the Finnish superheater deposits differed from the Brazilian deposit samples. Due to the different chemical composition and ergo lower amount of melt, the Finnish deposits formed a skeletal morphology and the dominating ageing mechanism was diffusional transport of alkali chloride vapours toward the steel. Two deposit archetypes were observed. Type 1 deposits had an innermost layer concurrently enriched in Cl and K. The probe deposits showed that a Cl and K-rich layer was formed also directly on the steel surface, where it subsequently caused corrosion via active oxidation. The local enrichment in alkali chloride was also identified to cause a local decrease in the first melting temperature of the deposit and reached a minimum in the Cl and K-rich layer that was formed directly on the steel surface. Furthermore, the probe deposits showed that over time, an overall enrichment in the average Cl content within the deposit had occurred. Alkali chlorides in the flue gas diffused into the porous deposit, where they subsequently condensed as the local temperature decreased. Type 2 deposits were characterized by a region enriched in K and S while concurrently depleted in Cl, due to sulphation. A deposit region initially enriched in alkali chloride, was sulphated over time as SO2 from the flue gas can diffuse into the deposit and subsequently react with alkali chloride. The initial melting behaviour of synthetic ash deposits was studied on a laboratory scale to identify the parameter determining the change of deposit morphology from skeletal to densely molten. The amount of melt formed in the deposit was identified to be the parameter determining the final deposit morphology. The transition from a skeletal to a molten deposit morphology is in this thesis suggested to take place at a melt fraction of about 30 wt-%. In deposits with lower melt fractions, the melt was observed to accelerate sintering and the formation of the skeletal morphology. The identified ageing mechanisms were also observed in synthetic deposit mixtures comprising NaBr Na2SO4 and KBr-K2SO4. This implies that the mechanisms are also applicable to other processes than kraft recovery boilers. Overall, the results of this thesis demonstrate the impact deposit ageing can have on the local chemical composition and morphology of actual superheater deposits. Independently of the final deposit morphology, the local first melting temperature of the deposits was shown to decrease toward the steel, which can promote the formation of melt in the direct vicinity of the steel surface and cause severe melt-induced corrosion. In addition, the formation of a layer of alkali chloride directly on the steel surface can promote corrosion via active oxidation. Furthermore, the results show that deposit ageing is not exclusive to kraft recovery boiler deposits but is also of relevance to deposits in other combustion processes.