Environmental assisted cracking of pipeline steels in CO2 containing environment

Buried pipelines are susceptible to Environmental Assisted Cracking (EAC) in three different conditions: Stress Corrosion Cracking in the presence of Carbonate-Bicarbonate (CB-SCC), Near Neutral Stress Corrosion Cracking (NN-SCC) and Hydrogen Embrittlement (HE). Both CB-SCC and NN-SCC involve in their mechanism the presence of CO2 and/or its dissociated species dissolved in moisture under the coating, while the generic term of HE refers to the phenomena of brittle fracture taking place for entry of atomic hydrogen inside the metal lattice, owing to the applied cathodic protection. Historically the CB-SCC was the first established form of stress corrosion on pipelines in the 60s, while Trans Canada Pipeline published in 1985-86 the first case of NN-SCC. The initiation conditions and the mechanism of propagation of CB-SCC were well established by means of the works of Parkins in 70ths. Many studies were carried out on NN-SCC, but actually there is not an exhaustive understanding of the problem. This paper summarizes the results obtained by authors on pipeline steels in NN-SCC promoting environments. Constant load, constant deformation, slow strain rate, slow bending and corrosion fatigue tests were executed on different grades of pipeline steels. The obtained results pointed out the effect of continuously plastic deformation to observe the propagation of NN-SCC cracks. NN-SCC cracks are preferentially nucleated from localized attacks. Inside the pits, the decreasing of pH enhances the hydrogen ions reduction. The results of slow strain rate tests are in agreement with a hydrogen embrittlement mechanism for the NN-SCC cracks propagation. Electrochemical tests, potentiodynamic and cyclic voltammetry were also carried out in order to analyze the effect of temperature, pH, CO2 and bicarbonate concentration on the pitting initiation. A pre-corrosion procedure, using a solution with high concentration of bicarbonate ions saturated with CO2 and a great number of voltammetry cycles was developed to obtain localized attacks on the specimen surface, similar than those observed in the failure analysis of the cracked pipelines. The SSR and slow bending tests carried out on these pre-corroded specimens evidenced presence of cracks with the same morphology of NN-SCC. Finally some corrosion-fatigue tests carried out on linear elastic fracture mechanics specimens (Single Edge Notch three four point Bending specimens) evidenced the increasing of fatigue propagation crack growth in NN-SCC environment. In the range of traditional and innovative pipeline steels, the mechanical properties (ultimate tensile strength and yield strength) seem do not influence their NN-SCC resistance

La realcalinización y la extracción electroquímica de los cloruros en las construcciones de hormigón armado

Realkalisation and electrochemical chloride removal techniques, developed for rehabiliting carbonated and chloride-containing structures, are presented. Electrolysis and electromigration mechanisms and consequences as well as electrochemical conditions at the reinforcement surface are discussed and compared with cathodic protection ones. Furthermore, possible side effects are commented

Environmentally assisted cracking of pipeline steels in CO2 containing environment at near-neutral pH

The paper summarizes the results obtained by authors concerning near-neutral stress corrosion cracking (SCC) phenomena on buried pipelines. Stress corrosion tests according to different methodologies and fatigue propagation tests were executed on traditional steel grades. Tests were performed also on specimens precorroded according to a procedure developed to produce localized attacks similar to those observed during failure analysis on pipelines, acting as preferential sites for crack nucleation. Electrochemical tests were carried out in order to evaluate the influence of environmental parameter on pit formation. The main role of continuous plastic deformations on near-neutral SCC is confirmed. The effect of pH, CO2, and bicarbonate concentrations on pitting formation is discussed

Experiences on corrosion inhibitors for reinforced concrete

Corrosion of carbon steel reinforcement is the most important cause of premature failure on reinforced concrete structures. Prevention of corrosion is primarily achieved in the design phase by using high quality concrete and adequate cover. Additional prevention methods are adopted when severe environmental conditions occur or on structures requiring very long service life. Among these methods, corrosion inhibitors seem to offer a simple and cost effective prevention technique. They may be used both as a preventative techniques, if added to fresh concrete, and as a repair system, if applied on hardened concrete. The performance of corrosion inhibitors for reinforced concrete structures affected by chloride induced and carbonation corrosion has been studied at PoliLaPP, Laboratory of Corrosion of materials "P. Pedeferri" of the Department of Chemistry, Materials and Chemical Engineering "G. Natta", Politecnico di Milano, in the last 15 years. Organic commercial admixed corrosion inhibitors delayed the occurrence of chloride induced corrosion. This result is related to two effects: reduction of the rate of chloride transport into concrete and increase of the critical chloride threshold. Commercial migrating corrosion inhibitors (MCI) are able to delay time-to-corrosion of passive rebars in concrete subjected to chlorides ponding; this effect is mainly related to the reduction of chlorides diffusion coefficient. MCI can reduce the corrosion rate after corrosion initiation only in carbonated concrete, nevertheless efficiency is low and residual corrosion rate is not negligible. Commercial MCI can penetrate into concrete mainly through capillary sorption but penetration depth is limited to 20 mm. The results obtained with nitrite-based inhibitor confirm literature data: the inhibitor is effective if the molar ratio [NO2 ]/[Cl ] is higher than 0.5-0.6; in carbonated concrete, due to insufficient amount of inhibitor, no significant effect has been observed on corrosion rate. Among the tested organic substances, compounds containing carboxylic group showed the best results solution tests: pitting potential, time-to-corrosion and critical chloride content are similar to those obtained with sodium nitrite. In concrete tests, only one amine and one amino acid showed good performance increasing the critical chlorides threshold with respect to the reference condition. For carboxylate substances, a strong link was found between inhibiting properties and molecular structure