Critical analysis of hydrogen permeation techniques. Application to different steel microstructures

The hydrogen diffusion understanding has assumed great importance due to hydrogen embrittlement related problems. This thesis has investigated, with electrochemical permeation tests, the hydrogen diffusion into API 5L X65 pipeline steel with three metallurgical microstructures. It has a dual purpose: • critical analysis of hydrogen permeation techniques; • the application to three different microstructures. The main results are listed below. The charging transient analysis (Devanathan and Stachurski's method standardized in ISO 17081) gives an apparent diffusion coefficient Dapp strongly influenced by metal-hydrogen trapping effect and cathodic surface electrochemical alteration. Indeed Dapp values are affected by high dispersion, they are insensitive to microstructural variations and they result at least one order of magnitude lower than lattice diffusion coefficient. Diffusion coefficient measured during the first part of discharge is sensitive to microstructural variations, it is less influenced by trapping and electrochemical alterations, it is higher than Dapp and only slightly lower than DL estimated with other procedures. Cathodic current partial charge and discharge procedure (Zakroczymski's method) shows less experimental data dispersion, it gives experimental curves perfectly symmetric (intrinsic reproducibility) which follow diffusion Fick's laws. The obtained D values can be reasonably assumed as "true" lattice diffusion coefficient of materials. These values are sensitive to microstructural variations. Martensitic (quenched) X65 steel has DL about 3 times lower than bainitic (quenched and tempered) X65 steel and about 14 times lower than ferritic pearlitic (annealed) X65 steel. In conclusion, the electrochemical experimental procedure adopted in this project consist of: 1) Dapp estimation during hydrogen charge; 2) DL estimation during hydrogen partial charge and discharge; 3) DL verification and hydrogen content estimation (in lattice and reversibly trapped sites), during hydrogen discharge. This procedure leads to reliable and meaningful results in permeation phenomena.