Patent GB-960346-A (GB)
Title: Method of treating metal surfaces and application of the method
IPC: C23C22/62
Filed: 1963-01-03
Abstract: Metal surfaces are pre-treated with an inhibited acid, further treated with an oxidizing and complex-forming solution and posttreated with a passivating solution.  The invention is applied to the cleaning and passivation of metal surfaces in nuclear reactor circuits such as heat-exchanger surfaces.  In circulatory systems an inhibited acid is circulated e.g. HCl or H2SO4, containing an inhibitor and, optionally, a compound providing fluoride ions in solution e.g. HF: this acid cleaning follows degreasing with an alkaline cleanser and removes oxides and foreign inclusions.  After rinsing, oxidizing and inhibiting chemicals, such as sodium nitrite, and iron-complexing chemicals such as citrates, tartrates, chelates or others are together circulated with distilled water, the pH is adjusted to a value over 9 with ammonia and the solution heated to 40 DEG C.: this treatment removes strongly absorbed acid anions, e.g. Cl ions, from the cleaning pre-treatment by neutralizing traces of acid from scores and crevices. Passivation treatment follows containing only elements which have neither a larger neutron capture cross-section nor induce substantial radio-activity so that the passivating coating contains no neutron absorbing materials, these causing strong radio-active rays of considerable duration: such suitable elements are H2 N2 O2 or C or compounds thereof e.g. carbonic acid, ammonia, nitrites and nitrates, amines, cyanides, carbonates; the isotopes of which have small radio-active radiation and/or short half-lives.  The elements S, P, Si and derivatives thereof e.g. sulphates, sulphites and sulphides, phosphates, phosphites, silicates; are less desirable but usable since the radiation danger from them is still insignificant.  Ca and Cu have radiation capacities causing small disturbance to operation but Na and Cr have strong radiation properties.  Of the materials tested specifically for passivation action nitrites are the most suitable; the passivation treatment thus comprises a solution of distilled water, organic nitrite and organic base which is pumped round and drained off. This phase of the treatment may be repeated if the [Na+] or other undesirable ions does not reach the prescribed low value of e.g. < 10mg/litre in the passivating solution.  A drying process follows the passivation treatment using heated air or inert gas depending on the drying time. The organic passivating solution may be a solution of dicyclo hexylamine nitrite and hydrazine (contains only non-dangerous isotopes) in distilled water.  The treated installation is stored under a controlled atmosphere. The passivating additives must have sufficient vapour pressure at normal temperatures to render their removal by gas-scavenging, preferably with the operating gas itself, before operation starts.  The passivating coating may be 30-50m  thick in sealed systems protected with a controlled atmosphere or 100m  thick in non-sealed systems.  When an installation is commissioned for operation a permanent protective coating e.g. of magnetite, can readily form on the clean metal surface under normal operating conditions.  Heat circuits of the kind described are made of high-grade alloy steels, Al and Al alloys, or unalloyed or low-alloy structural steels.