Field history has shown that a modified baked on phenol-formaldehyde plastic coating can be successfully phenol-formaldehyde plastic coating can be successfully used in hydrogen sulfide environments, although laboratory tests indicate 5% HgS, 2,000 psi and 200 deg. F to be the upper limits for satisfactory performance. A theory for this behavior is advanced. Application methods and special handling procedures for this material are reviewed.
The success of phenolic coatings in H2S environment is the result of combining certain inert fillers with thermo-setting phenolic resins. The type and amount of filler used in phenolic coating materials influences the temperature limit, chemical resistance, flexibility permeability and intrafilm gas diffusion rate through the coating film.
The degree of success in any case is dependent upon the environmental temperature, pressure and concentration of the corrosive gases, H2S and CO2.
A paper entitled "Iron Sulfide Coatings to Reduce Hydrogen Damage in H2S Environments" by Dautovich and Hay was presented at the NACE Corrosion '77 Conference in San Francisco. It stated that the application of a sulfide barrier shows great promise as an operating practice to protect industrial components from hydrogen damage. protect industrial components from hydrogen damage. The iron sulfide is said to behave as an ionic barrier. In order to perform in this manner, however, there must be a uniform coverage of the metal. Any void area could result in a hydrogen sulfide corrosion cell. In downhole conditions this coating is theorized to function as a diffusion membrane allowing only a thin uniform iron sulfide layer to be deposited and subsequently keeping this layer mechanically intact.
Conventional baked on phenolic coatings often blister when gases and vapors attempt to escape from behind the film at a rate greater than the normal diffusion properties of the coating.
The special modified phenolic has inherent high gas diffusion rates, producing coating system that allows unreacted gases and vapors to escape without blistering the coating while providing adequate corrosion protection for the steel.
Coating films prevent corrosion by restricting the availability of corrosive material to the metallic surface. Coatings are, therefore, barrier materials. Basically, a barrier needs cohesiveness, resillence and strength. In addition to forming a barrier, a coating may possess other characteristics to further control corrosion, such as corrosion inhibiting pigments. These pigments contribute strength to the barrier and may also be made to regulate the pH of the substrate environment thus retarding corrosion.
Under high pressure conditions at temperatures above 200 deg. F in sour service, coatings change character and become low order barriers. The coating has an increased degree of permeability in the presence of H2S. The penetration of the coating results in the formation of a thin iron penetration of the coating results in the formation of a thin iron sulfide film and a nightly decreasing adhesion to the steel substrate. While the coating film remains intact, the iron sulfide layer beneath the coating film is self limiting and corrosion protection exists.
Baked phenolic formulations have varying gas diffusion rates which limit the preferred pressure blow-down rate of the tubing string. Releasing internal tubular pressure at it rate higher than the gas diffusion rate of the coating will result in fracture of the coating film.
Removal of a coated string from service should, therefore, be handled with care.