The article concerns the experimental testing of the long-lasting effect of surrounding environmental conditions (high air humidity and neutral salt mist) on the characteristics of the fatigue service life of T-joints in structures made of low-alloy steel 15HSND. The article-related research involved metallographic tests of the weld and of the heat affected zone (HAZ) of welded joints subjected to the 1200-hour long effect of humidity (in the G4 humidity chamber) and salt mist (in the KST-1 salt mist chamber). The metallographic tests made it possible to calculate the degree of corrosive effect and the entire area affected by corrosion, the depth of corrosion stain and corrosion pit penetration in the surface layer of the metal of the fillet welds and the HAZ of T-joints. The research also involved the identification of the characteristics of the fatigue service life of welded joints subjected to the long lasting effect of a corrosive environment.