hello all !
The issue was a cap on hydraulic tank, fixed with only 2 screws in 2mm thickness steel sheet, lets the rain get into tank
To fix that I chose to increase steel thickness (2mm -> 6mm) and number of screws (2 -> 4). the cap is not weldable, flanged way is chosen.
1 - drawing
2 - FEM analysis
3 - change drawing and select suitable steel
4 - machining, welding, drilling, threading
Well done ! FreeCAD is cool