Thin layers of black phosphorus attract a growing interest in electronics and optoelectronics for their bidimensional properties (2D) giving raised to high mobility and tunable direct band gap. Thin films are however unstable in air and synthesis of high quality samples consisting of pristine few-layered materials remains challenging, as a result. We recently established that a combination of oxygen, light and moisture provides the conditions leading to a photo-oxidation of the layers. In controlled conditions, the kinetic of the reaction is well captured by a redox model, but the evolution is less clear in normal humidity conditions where water can simply accumulate at the surface and forms droplets. In the present study, we investigate the wetting properties of thin black phosphorous films and its evolution during degradation using FastScan atomic force microscopy in different humidity conditions. From a statistical analysis of the bubbles formed, the wettability is found to decrease with layer thickness. This difference in surface tension between thinner and thicker layers is ascribed to an accumulation of phosphoric acid in the bubbles, which reveals increasing degradation rates with decreasing layer thickness. From our analysis and a simple rate equation model, it is found that: i) A threshold humidity is necessary for water condensation; ii) The photo-oxidation occurs on single bubble sites; iii) Black phosphorus layers immersed in water slowly thickens and crumbles anisotropically due to water etching during degradation.