Porosity in the e sublayer, as developed during nitriding of an a-Fe substrate at 570 C (843 K) for 7 h at rN = 1.93 atm1/2, by the association of dissolved nitrogen as N2 gas molecules at grain boundaries in the sublayer but also within the grains of the sublayer. The porosity is most pronounced in the surface-adjacent part of the sublayer, because this is the oldest part of the sublayer and this part of the sublayer has the largest dissolved nitrogen content, implying a larger driving force for N 2 gas formation close to the surface than at larger depths. The pressure in the pores is so large that local distortion at the surface leads to local bulging out of the nitrided material. The developing pores at the grain boundaries can coalesce in advanced stages of nitriding, leading to “open” grain boundaries in contact with the outer nitriding atmosphere (see also Fig. 9b and its discussion in Section 9). Light optical micrograph of cross section; etched in 1 vol% Nital; oblique illumination, green light. Courtesy of M.A.J. Somers