Cathodic protection requirements for high strength steel in sea water assessed by potentiostatic weight loss measurements

Abstract

A potentiostatic weight loss technique has been used to compare the corrosion rates of Weldox 700 high strength low alloy steel exposed to both natural and sterile sea water at potentials ranging from the free corrosion condition to - 1000 mV (SCE). Anodic polarisation curves with Tafel constants of 54 and 64 mV per decade were fitted to the data and the minimum levels of protection required to limit the corrosion rate to 0·001 mm per year were found to be - 770 and - 790 mV (SCE) in natural and sterile sea water, respectively. Lower corrosion rates occurred in filtered natural sea water than in sterile sea water but the highest rates were recorded on specimens ex posed to a seabed sediment. The reasons for this behaviour are discussed in terms of the biofilms and the numbers of sulphate reducing bacteria present on the surfaces.     

Effect of different offshore seabed sediment on steel corrosion

A series of simulation experiments on corrosion behaviour of carbon steel (A₃ Steel) and low alloy steel (16Mn steel) in marine atmosphere (MA), seawater (SW) and seabed sediment (SBS) including rough sea sand, fine sea sand and seabed mud were carried out indoors for a year or so by means of individually hanging plates (IHP) and electrically connected hanging plates (ECHP). Results showed that the corrosion rates of A₃ and 16Mn steel in marine environment were in the order: MA > SW > SBS by the IHP method, MA > SBS > SW by the ECHP method. The corrosion rates of steels in the water/sediment interface were directly proportional to the grain size of the SBS by the ECHP method, but those of steels in the water/sediment interface did not vary with the grain size of SBS by the IHP method. The corrosion rate of low alloy steel was a little higher than that of carbon steel. The corrosion in seabed sediment was mainly determined by the macrogalvanic cell (oxygen concentration difference cell) between steel plates in different sediment and depth.     

阴极保护电位对E550钢氢脆敏感性的影响

采用慢应变速率拉伸实验方法 (SSRT) 结合断口扫描电镜 (SEM) 观察,研究了阴极保护电位对E550钢在海水中氢脆敏感性的影响.结果表明:随着阴极保护电位负移,E550钢在海水中的氢脆敏感性增加,阴极保护电位为-0.95 V (vs SCE) 时,拉伸试样出现脆性解理断裂特征,电位为-1.05 V时,E550钢断口呈明显脆性断裂特征.     

Technical note: Effect of metal temperature on current density requirements for cathodic protection of bare steel in sea water: laboratory study

Abstract

The production. of oil and gas from deeper seas and deeper wells means that crude oil is now to befound at higher temperatures than previously experienced. It is expected that oil from wells with bottom hole temperatures of up to 300°C will be produced in thefuture. To model the effect of elevated external skin temperature on the effectiveness of the cathodic protection of an uncoated steel riser, three sea water cells were used, each of which consisted of a fabrication comprised of a simulated riser housing a cartridge heater, a sacrificial anode, and measuring instrumentation. After a preliminary aging period under cathodic protection at low temperature to allow a calcareous deposit to form on the substrate, the heaters were switched on and the specimens were subjected to temperatures in the range 40–90°C, and the applied current density to achieve a suitable level of cathodic protection was recorded. The results, which are discussed in the context of previous studies, show that there is no simple and reliable relationship between the operating temperature of a surface and the required current density for cathodic protection in sea water. Further work needs to be undertaken to produce a more reliable model for the determination of necessary cathodic protection densities.     

Effect of applied potential on hydrogen embrittlement of weld simulated HSLA–80 steel in sea water

Abstract

Hydrogen embrittlement behaviour of an HSLA–80 steel in the weld simulated, grain coarsened heat affected zone condition, in synthetic sea water under cathodic charging in the applied potential range of -600 to -1400 mV(SCE) has been studied using a slow strain rate technique. Loss of ductility, as reflected in the percentage elongation and percentage reduction in area values, was substantial at and beyond -800 mV(SCE). The material in the weld simulated condition showed more susceptibility than the as received material, which is considered to be a result of increased strength and a bainitic–martensitic microstructure. Fracture showed both microvoid coalescence and quasi-cleavage features and was indicative of hydrogen induced void nucleation. Both hardening and softening effects on hydrogen charging were experienced.     

船用5052-O铝合金在海水中的力学和电化学性能(英文)

采用电化学实验和SEM表面形貌观察,对船用5052-O铝合金在海水中的腐蚀保护电位进行优化,以克服诸如点蚀、腐蚀、应力腐蚀开裂和氢脆等行为的发生。在外加电流阴极保护的条件下,最优的保护电位范围为-1.3V～-0.7V。在此电位下,试样的腐蚀电流密度较低,经恒电位实验后,试样表面形貌保持得较好。     

Cathodic Protection of Steel Embedded in Porous Concrete

Abstract

The feasibility of applying cathodic protection to carbon steel strands embedded inporous concrete exposed to a chloride-containing environment was investigated. In-situ polarisation experiments were conducted on the embedded steel using a current-interrupt technique. This enabled the magnitude of the iR drop to be determined, and the actual (or corrected) potential of the embedded steel to be monitored throughout the experiments. The results suggested that, for the porous concretes which were examined, the minimum cathodic potential required to protect the embedded steel depended on the concentration of chlorides in the environment. These potentials are ?260 mV SHE (?578 mV CSE) for a 0·02 M NaCl environment and ?410 mV SHE (?728 mV CSE)for 0·7 M NaCl or sea water solutions.     

Cathodic protection of water-line areas

Abstract

The effect of cathodic protection on intermittently immersed water-line areas has been studied. Corrosion was greatly reduced in sea water compared with that in sodium chloride solution, and the degree of protection was greater at higher frequencies of immersion. The protective effect of cathodically deposited ‘chalk’ is discussed. No corrosion occurred at a static water-line in sodium chloride solution.     

Critical questions and answers about cathodic protection

ABSTRACT

Cathodic protection has been in use for almost two hundred years, yet there still exist a number of misunderstandings surrounding its application. In this paper, we question some common statements and beliefs to determine whether they are based on fact or fiction. Our questions include: What is the origin of the ubiquitous ?0.85?V_CSE cathodic protection criterion and why does it work? Does cathodic protection really stop corrosion or simply reduce it to negligible levels? What does the instant off potential tell us and is it really the true polarised potential? Does hydrogen gas cause cathodic disbonding? Do the potential shifts measured during interference testing indicate real interference? The answers to these questions are important because they allow the correct interpretation of field and experimental data, based on a true understanding of the electrochemical and thermodynamic basis of cathodic protection.     

Effect of cathodic polarization and sulfate reducing bacteria on mechanical properties of different steels in synthetic sea water

At the slow strain rate tensile tests done using the specially designed facility, the decrease in the elongation to fracture, reduction of area, fracture energy and no effect on the strength have been stated for the low alloy ferrite‐pearlite and sorbite steels, polarized in synthetic sea water at potentials corresponding to the cathodic protection (? 800 to ? 1400 mV_SCE). Presence of SRB promotes the plasticity loss, being especially pronounced at potentials ? 1100 to ? 1200 mV_SCE. At higher cathodic polarization, the plasticity estimated in inoculated and in sterile water equalizes. The effects have been correlated with the contents of absorbed and of permeable hydrogen. The promotion of hydrogen charging and the plasticity loss by SRB at the low and medium applied cathodic polarization has been accounted for the observed production of S^?2 ions and inhibition of deposit formation. The negligible effect of SRB at the high cathodic polarization has been suggested to be a result of the suppression the SRB growth due to the high alkalization of the near surface solution. The same amount of hydrogen produces the less detrimental effect on the sorbite than on the ferrite‐pearlite steel. However, at the similar cathodic polarization, the sorbite steel absorbs the highest amount of hydrogen and reveals the most pronounced degradation. Cathodic protection of constructions subjected to the action of SRB in the sea water should provide the conditions, under which no fragment of marine construction could be polarized by potential corresponding to the maximum degradation of the plastic properties of steels (? 1100 to ? 1200 mV_SCE).     

Cathodic protection of UNS C71500 heat exchanger tubes in sulphide polluted Arabian Gulf sea water

Abstract

A laboratory investigation using a specially designed circulating test rig has been undertaken to study the feasibility of achieving complete cathodic protection of UNS C71500 heat exchanger tubes in sea water applications by means of sacrificial carbon steel anodes. The results have indicated that the galvanic current distribution covered the entire 6 m length of the tube. However, the presence of sulphide ions as pollutants in the sea water shifted the galvanic potentials of the tubes to less active potentials and affected the development of the protective films that normally form.     

Hydrogen uptake by structural steels at cathodic protection in sea water inoculated with sulfate reducing bacteria

The effect of sulfate reducing bacteria (SRB) on the hydrogen permeation rate through ferrite‐pearlite and sorbite steels of quite similar chemical composition was studied using a specially designed facility. Tests were carried out in synthetic sea water, sterile or inoculated with bacteria, at potentials corresponding to cathodic protection (? 800 mV to ? 1400 mV_NCE). Cathodic polarization within the studied potential range did not stop the metabolism of SRB. Presence of SRB was found to increase the hydrogen permeation rate, to form S^2? ions, to increase the polarization current, to modify the impedance spectrum and to change the appearance of cathodic deposits in comparison with sterile conditions. The promoting effect of SRB on the hydrogen uptake was concluded to be the result of the increase in polarization current due to the formation of the less protective layer of cathodic deposits on the steel surface, the presence of S^2? ions and the possible decrease in pH. Despite the similar tendencies, the effect of SRB on hydrogen uptake was more pronounced in the case of sorbite steel. The bacteria action can cause hydrogen deterioration of steel at potentials, recognized as safe ones at cathodic protection.     

Constant strain-rate testing of AISI type 304 stainless steel in 0.5 M H2SO4 + 0.1 M NaCl solution at room temperature with controlled potential

The constant strain-rate (CSR) test method has been applied to investigate the stress corrosion cracking (SCC) susceptibility of AISI Type 304 stainless steel in 0.5 M H₂SO₄ + 0.1 M NaCl at 25°C within a range of controlled potentials. Cracks were found to form in two distinct potential regions: in the cathodic range, and in the passive potential region. A clear differentiation between the two observed types of cracks was attempted. Crack morphology, the measured current during straining and the reduction of area were used as criteria. SCC was found to occur essentially in the passive potential region; in the cathodic range the cracks were due to hydrogen embrittlement.     

Characterisation of calcareous deposits on freely corroding low carbon steel in artificial sea water

Abstract

Calcareous deposits were formed on steel under cathodic protection conditions in artificial sea water at various potentials from ?0·900 to ?1·400 V(SCE). The deposition calcareous layers were characterised by electrochemical impedance spectroscopies, scanning electron microscopy observations and X-ray diffraction analyses. At 20°C, the deposits were composed of calcite CaCO₃ when formed at various potentials in solution 1, of brucite Mg(OH)₂ and aragonite CaCO₃ when formed at potentials from ?0·900 to ?1·200 V(SCE) and only of brucite when formed at potentials E??1·300 V(SCE) in solution 2. Magnesium seems to influence the corrosion behaviour of freely corroding steel by causing calcium carbonate to precipitate as aragonite. Aragonite is more effective in covering the surface than calcite and is therefore more functional in preventing oxygen from reaching the steel surface, thereby lowering the corrosion rate.     

Einfluß der Festigkeit unlegierter und niedriglegierter Stähle auf H-induzierte Korrosionserscheinungen bei zügiger plastischer Belastung

Influence of mechanical strength on hydrogen-induced corrosion effects on unalloyed and low-alloysteels subjected to slow strain rate tests Three steels were subjected to various heat treatments or cold working to produce 7 variants of strength. Specimens from these materials were tested in 9 aqueous solutions containing various acidic components by the constant strain rate technique under cathodic polarisation or free corrosion conditions. Hydrogen induced cracks were only observed after passing the maximum load. Hydrogen induced cracking and the decrease in the reduction of area (hydrogen embrittlement) show a good correlation with the mechanical properties of the materials. For less ductile materials the number of cracks decreased with increased embrittlement. The magnitude of hydrogen embrittlement depends on the concentration of undissociated acid in the test solution and is independent of pH value. O₂ can reduce the embrittlement. The effect of test solution composition decreases as the cathodic polarisation potential becomes more negative, because hydrogen will then be generated from H₂O. There is no correlation between the type of corrosion effects and the strength of the material, except in the case of highly sensitive high strength variants with R_m in excess of 1000 N/mm². Quenched and tempered low alloy steels, even at high strength levels, have significantly higher resistance to hydrogen embrittlement than significantly higher resistance to hydrogen embrittlement than unalloyed steels. The same is also ture for workhardened variants. For unalloyed steels, metallurgical cleanness seems to have a favourable influence. No hydrogen induced corrosion effects were observed in specimens tested at 907°C under free corrosion conditions.     

Electric field of cathodically protected pipeline

Abstract

The electric field caused by the cathodic protection of a buried pipeline, both on the surface and in the soil surrounding the pipeline is considered; at every point the value of the electric potential function U _(x,y,z) is computed from which the electric field intensity E caused by the cathodic protection system is established. To obtain these results, both the pipeline and the anodes are represented by spherical elements. The precision of this computational approach is validated experimentally. The results are presented as curves and lines of electric field.     

Effects of polarisation on mechanical properties and stress corrosion cracking susceptibility of 7050 aluminium alloy

The effects of polarisation (including cathodic and anodic polarisation) on mechanical properties and stress corrosion cracking (SCC) susceptibility (I_SCC) of 7050 aluminium alloy have been investigated by means of polarisation and slow strain rate test. The results of cathodic polarisation experiments showed that the I_SCC increased with shifting negatively the polarisation potential when the cathodic potential E_C≧?1100 mV(SCE), while it decreased with shifting negatively the polarisation potential when the cathodic potential E_CI_SCC increased severely with increasing the polarisation potential. In addition, the extents for the effect of polarisation potential on I_SCC were different among the 7050 aluminium alloy under various aging states. Polarisation was of the biggest effect on the I_SCC of under aged state, the smallest effect of over aged state and the middle effect of peak aged state. The SCC mechanism of aluminium alloy was a combination of anodic dissolution and hydrogen embrittlement, and the effects of hydrogen on SCC increased with increasing the hydrogen concentration.     

Measurements of hydrogen activity in iron during cathodic protection using a potentiometric concentration cell

A solid - state potentiometric sensor, based on the well - tried principle of the thermodynamic concentration cell, has been constructed. The sensor uses a conventional bi-electrode design with an entry side and an exit side at which the hydrogen is detected. The sensor has been used for the quantitative determination of the hydrogen activity (equivalent H₂ pressure) generated during cathodic protection. Thus, the background hydrogen pressure in uncharged steel is 10^?17.7 (2 × 10^?18 atm.) with a variation of about 20 times. During cathodic protection this value rises by large amounts. Thus, using a zinc anode, the equivalent hydrogen pressure rose by 10^10.3 (to 4.4 × 10^?8 atm.) in 3.5% NaCl and by 10^16.9 (to 0.16 atm.) in artificial sea water, indicating the much larger amounts of hydrogen present in the latter case. This compares, for example, with literature data of 0.11 μA cm^?2 and 0.60 μA cm^?2 for hydrogen permeation at ?1000 mV (SCE) in 3.5% NaCl and artificial sea water respectively. Hydrogen entry and exit was also considerably slowed in sea water. These differences are caused by local surface pH buffering and deposit formation.     

Hydrogen embrittlement of duplex stainless steel under cathodic protection in acidic artificial sea water in the presence of sulphide ions

By slow strain rate technique, hydrogen embrittlement (HE) of a 2205 duplex stainless steel was studied in deaerated acidic (pH 6.5) artificial sea water, in the absence and in presence of sulphide ions (1-30 ppm). Strain rate tests (1 × 10⁻⁶ s⁻¹) were performed on specimens polarized at −0.9; −1.0 and −1.2 V_SCE at 25 ± 0.1 °C. HE was evaluated by R, the ratio between the % elongation to fracture in the aggressive solution and in air.Duplex stainless steel were subjected to HE in acidic artificial sea water at −0.9 V_SCE. HE increased at −1.0 V_SCE but it was reduced at −1.2 V_SCE. This decrease was attributed to the influence of a calcareous deposit.Sulphide ions at 1 ppm were sufficient to stimulate HE of duplex stainless steel. The higher the sulphide amount and the more negative the cathodic potential, the higher HE was. In the presence of S²⁻, the shielding effect of the calcareous deposit was not evident.     

Stress-induced failure of high-strength steels in environments containing hydrogen sulphide

Abstract

The pre-cracked cantilever beam test is a sensitive means of evaluating the relative susceptibilities of different steels to stress-Induced failure in H₂S-containing environments. This, test indicated that a 12Ni 5Cr.3Mo maragmg stee1 is considerably more resistant to stress-induced crackining than the low-alloy steels 0Ni 1Cr 2Mo and 2Ni 1Cr 3Mo. For low-alloy steels with no applied potential the failure appears to be caused predominantly by hydrogen embrittlement. Impressed anodic or cathodic potentials have no effect on the time to Failure of low-alloy steels where as a zone of immunity exists for maraging steel with Impressed potentials Within the approximate range ?0·6 to ?1·0 V. This suggests that the maraging steel is less susceptible to hydrogen embrittlement than the quenched and tempered low-alloy steels. Permeability studies indicate that stress-induced failure is related to the ability of the steel to transmit hydrogen and to the nature of hydrogen traps in the steel.