Improvements in or relating to the protection of steel from hydrogen damage

摘要

844,227. Preventing corrosion. INSTITUT FRANCAIS DU PETROLE, DES CARBURANTS ET LUBRIFIANTS. July 7, 1958 [July 5, 1957], No. 21762/58. Class 123(1) Damage to steel structures immersed in an aqueous corrosive medium, particularly one containing H 2 S and/or HCN, by fissuring, blistering and embrittlement due to hydrogen absorption is prevented or reduced by first adjusting, if necessary, the pH of the medium to between 4 and 10, preferably to between 7 and 8, and then applying a cathodic current of a low, predetermined value to the structure. The value of the current is found by immersing a number -of samples of the structural steel in the same pH-corrected corrosive medium under the same corrosion conditions. The fragility as determined by a tensile strength test and/or the number of surface flaws per unit area found for each sample after unit time of application thereto of a particular cathodic current is plotted against the current density on the sample. The current applied to the structure is then set to give a current density thereon corresponding to that on the sample having the minimum fragility and/or number of surface flaws. The Figure shows the curve drawn for a number of steel samples immersed in water saturated with H 2 S and containing 2 mg. per litre of O 2 . The pH of the solution was adjusted to 6À5 with NaOH. The number of surface flaws per sq. dm., plotted as ordinates against the current densities reveal a current density range P 1 -P 2 in which the absence of flaws indicates that the hydrogen absorption is zero. The working current is accordingly set so that the structure current density falls within this range. The range may be widened by the addition of an organic inhibitor to the corrosive medium in the proportion of from 1 to 10 parts per million. Suitable inhibitors are aliphatic amines, quaternary ammonium salts, organic sulphides and disulphides, mercaptans and imidazolines.