The objective of the present study was to analyse the permeability of
hydrogen through the nitrided case of a high strength low alloyed stee
l. This type of thermochemical treatment is used to improve mechanical
properties, such as tribological behaviour and fatigue resistance. Hy
drogen contamination is intended to be avoided because of the risk of
hydrogen embrittlement. This may be achieved by the formation of a com
pact nitrided layer which plays the role of an efficient diffusion bar
rier for hydrogen. The substrate was a high strength low alloyed steel
, with a microstructure composed of banded pearlite in a ferrite matri
x which was plasma ion nitrided at 500 degrees C during three differen
t nitriding times : 0.5 h, 16 h and 28 h. Scanning Electron Microscopy
and X-Ray analysis were used to characterize substrate and nitrided c
ase microstructures. Microhardness profiles were obtained and then com
pared to microdeformation profiles determined by X-Ray diffraction.For
the sample nitrided for 0.5 h, the combination layer or white layer i
s irregular, measuring 2 to 3 mu m in thickness. The white layer is co
mposed of the gamma' (Fe4N) nitride. For the 16 h and 28 h of nitridin
g, the white layer is homogeneous with a thickness of 5 mu m. The whit
e layer is composed of gamma' nitride for 16 h of nitriding while in t
he 28 h of nitriding epsilon (Fe2-3N) nitride is present in the outerm
ost layer. The very high hardness attained in the combination layer is
due to the intrinsic hardness of the nitrides, while the higher hardn
ess in the diffusion layer is related to the presence of nitrides asso
ciate to the high dislocation density, the pinning of the dislocations
by precipitates as well as the saturation of the ferrite matrix. Elec
trochemical tests were performed to obtain potentiodynamic polarizatio
n curves and hydrogen permeation curves, at 50 degrees C, using a 0.1
N NaOH solution as electrolyte. This enabled the determination of hydr
ogen permeability, solubility and diffusivity in the substrate and in
the nitrided cases using De Miranda et al.'s methodology and Ash et al
.'s model. The three types of ion nitrided cases present different com
bination and diffusion layers as a function of the nitriding time. The
calculation of hydrogen permeation parameters in the different layers
allows a comparison of the behaviour of the cases which reference to
the microstructure and the mechanical properties. It is shown that, de
pending on the microstructure obtained after nitriding, the hydrogen p
ermeability may be reduced (for 16 and 28 h nitriding) or increased (0
.5 h nitriding) compared to that of the substrate. Variations of the p
ermeability are found to be essentially due to the presence and the th
ickness of the combination layer. Comparison of the permeabilities for
0.5 h and 16 h nitriding times shows that a critical thickness exists
for the white layer underneath which the role of barrier is not assum
ed. For 28 h nitriding time, the presence of epsilon (Fe2-3N) nitrides
as well as porosity into the white layer induce a decrease of hydroge
n permeability.