Corrosion by-products in steel transmission pipelines may vary in composition but will include common oxides such as ferric oxide. Otherwise, known commonly as rust.
Such by-products can be readily dissolved by common mineral acids, such as hydrogen chloride, according to the following reaction:
Fe2O3 + 6HCl → 2FeCl3 + 3H2O
Stoichiometrically, 1 kg of ferric oxide would require 1.4 kg of hydrogen chloride to fully dissolve. An equivalent amount is contained in 3.35 L of West Penetone’s TETRAGARD. Where needed or required, talk to your West Penetone representative for other product options.
This reaction occurs readily under aqueous conditions and would not be fully effective where oxide deposits are completely oil-wetted or heavily encapsulated in a hydrocarbon matrix. This method of cleaning would also require proper corrosion protection under atmospheric exposure where steel surfaces are being cleaned.
Iron sulfide deposition is another form of corrosion by-product that can vary widely in both stoichiometry and crystallographic forms leading to variable degrees of aqueous solubility. However, it is known that the more hazardous pyrophoric form, ferrous sulfide or troilite, can be readily dissolved by common mineral acids, such as hydrogen chloride, according to the following reaction:
FeS(s) + 2HCl(aq) → FeCl2(aq) + H2S(g)
This reaction occurs readily under aqueous conditions and produces large volumes of hydrogen sulfide gas as a by-product. This method of removal therefore requires not only proper corrosion protection but also the means to remove the produced gas either externally or in-situ.
Where gaseous by-products or solid precipitation create serious operational issues, other forms of removal include the use of chelates with the physical capacity to directly dissolve the scale deposits. One such chelate includes tetrakishydroxymethylphosphonium sulphate (THPS). This material requires additional ingredients for the complex formation process to occur.
Although the stoichiometry of the complex formation process has been quantified, recent studies have shown that it is not entirely predictive of uptake in the field due to varying rates of solubility of the different forms of iron sulfide. However, troilite (FeS) and pyrite (FeS2), both representing the near extremes of species formed, are expected to have a solubility of approximately 30 g/L or 0.25 lb/usg in 20% solutions of THPS and activator ingredient within a minimum contact time of 30 minutes.
Additional forms of removal also include abrasion under conditions of surface wetting and high pH. This method depends on some level of chelation but is largely contingent on keeping pyrophoric forms water-wetted and protected from air exposure for external treatment.
Generally, where low pH treatment is employed, corrosion protection can be achieved by pigging under a nitrogen atmosphere, following with a flush of soda ash solution, or by imparting a hydrocarbon film using a flush of diesel.
It is recommended that deposit density measurements as well as deposit characterization be completed and reported for reference in designing both the pig train components and chemical program. Otherwise, estimated scale volumes based on varying degrees of scale thickness can be used to estimate the total volume of descaling product required.
The composition of the pig train is vital on a mechanical standpoint. At a bare minimum, it is recommended that the type of pigs used be comprised of a utility, brush (e.g., pencil), and utility/poly plug pig to accommodate aggressive displacement of scale and subsequent debris. It should also be emphasized that any debris removed from the pipeline under the options noted above would need to be handled as hazardous waste, unless retain monitoring or fluid characterization post-application suggests otherwise.
