During casting of interstitial free steel, argon bubbles are caught in the solidification front of the steel slab. During downstream processing, these bubbles develop into sub-surface defects called blowholes or pencil pipe defects. Atomic hydrogen can enter the steel in the pickling line and accumulate in the defects, causing an increase of the internal pressure. In the presented study, blowholes are charged with hydrogen both electrochemically and by exposing the blowholes to pickling acid. The pressure and composition of the gas inside the blowholes are determined by opening the blowholes under ultra high vacuum conditions. It is shown that the accumulated pressure is capable of deforming the blowholes plastically, enhancing the visibility of the defects in the sheet. Calculations of the threshold pressure for plastic deformation show to be in reasonable agreement with measured pressures inside the blowholes. Industrial pickling conditions are discussed as well. It is concluded that hydrogen does not influence the blowholes significantly for the considered industrial conditions. Most likely the blowholes become visible in sheet steel at recrystallisation annealing. The final shape of the blowholes is determined by the temper rolling process.KEY WORDS: blowhole; pencil pipe defect; interstitial free steel; hydrogen; argon; pickling; surface defects.performed at room temperature using a 0.1 M NaOH solution. The sample served as cathode and a platinum grid positioned opposite to the sample acted as anode. A constant current density of 20 mA cm Ϫ2 was used. The hydrogen charging with an acid was performed using 20 mass% hydrochloric pickling acid. The acid is taken directly from the pickling line at Corus IJmuiden (including pickling inhibitor). The temperature of the acid was kept constant at 85°C.The electrochemical method was used to show deformation resulting from accumulation of hydrogen in the blowhole. Due to the low temperature and the lower efficiency (compared with pickling), the charging process is slow and therefore easy to control. Another difference is the charging time: the electrochemical method does not dissolve the steel surface and can be pursued for periods of one day or more. Because of etching effects the maximum pickling time is limited to about 10 min.Before and directly after charging, height profiles of the samples were taken using a UBM laser profile meter. This method gives a height resolution of 60 nm. Recording a height profile over an area of 4ϫ15 mm (with a lateral resolution of 20ϫ100 mm) takes about 30 min. Directly after taking the scan, the samples are stored in liquid nitrogen preventing hydrogen permeating out of the samples.The samples are analyzed by opening the blowholes in a closed vacuum chamber. After removing a sample from the liquid nitrogen, the sample is cleaned in ethanol, dried and placed in the above-mentioned ultra-high vacuum set-up. The sample is mounted in a sample holder shown in Fig. 1. The sample is kept in its place by a spring and a screw for adjustment....