Pitting,Crevice and stress corrosion resistance of high chromium und molybdenum alloy stainless steels

This paper presents new data on the resistance of recently developed high-alloy stainless steels to localised corrosion in chloride solutions. Pitting potential was determined in artificial sea water, and critical pitting temperature CPT in very aggressive FeCl₃ solution. Critical crevice corrosion temperature CCT was tested in the same FeCl₃ solution. Stress corrosion measurements, made in a more familiar NaCl solution by the drop evaporation method, demonstrate that alloy stainless steels with high chromium and molybdenum have very long failure times, comparable with those of nickel alloys found to be SCC-resistant under practical conditions. Stainless steels of 20 Cr 25 Ni 6 Mo type showed the best resistance to localised corrosion.     

Galvanic reactions during localized corrosion on stainless steel

During localized (crevice and pitting) corrosion, a local cell is established between an anode within a crevice or pit and a cathode on the surrounding passive surface. Data are presented to show that concentrated acidic chloride solutions, simulating corrosion product hydrolysis within a crevice or pit, produce potentials which are active (negative) to the normal surface passive potential. This behaviour explains the previously observed active drift of corrosion potential after initiation of crevice or pitting attack in dilute chloride solutions. The active state in concentrated chloride solutions was quite noble (positive) compared to the active state in more dilute solutions. Thus, there is no need to invoke ohmic resistance effects to account for the active state within a crevice or pit.Experiments were devised in which the local anode within a crevice was physically separated from the nearby passive-surface cathode. When the two were coupled together electrically, the cathode surfaces were polarized nearly to the unpolarized local anode potential, with only a few millivolts anodic polarization at the anode within the crevice. The rate of localized corrosion appears from the data to be limited by the rate of dissolved-oxygen reduction on the cathode surfaces. Thus, localized corrosion in dilute chloride solutions will be increased by (a) raising the temperature, (b) adding an oxidizer such as Fe³⁺ ions, or (c) substituting external anodic polarization for dissolved oxidizers.The overall potential, E_corr acquired by a specimen undergoing pitting or crevice corrosion is demonstrated to be near the protection potential, E_p below which pitting corrosion cannot propagate. Any potential active to E_corr and E_p results in cathodic polarization and suppression of the anode reaction in a crevice or pit. Since both E_corr and E_p vary with the extent of previous localized attack, E_p is not a unique property of the alloy as has been sometimes suggested and is of limited value in classifying alloy resistance to localized corrosion.     

Kühlwasserseitige Korrosionsbeständigkeit von metallischen Werkstoffen zur Handhabung von Schwefelsäure

Cooling water side corrosion resistance of high alloyed materials for handling of process side sulfuric acid The approved materials for use in sulfuric acid alloy 825 (German material No. 2.4858) and alloy 20 (German mater. No. 2.4660) have only a low resistance against localized corrosion in chloride containing water and are unsuitable for handling of sulfuric acid. The newly developed austenitic Cr-base alloy, alloy 33, (X1CrNiMoCuN 33-32-1, German mater. No. 1.4591) with 33 % Cr, 31 % Ni, 0,6 % Mo and 0.4 % N should have an excellent resistance against pitting and crevice corrosion additional to its high sulfuric acid resistance, too, because its Pitting Resistance Equivalent No. calculated according to PREN = %Cr + 3,3 · %Mo + 30%N runs to 50. Pitting and crevice corrosion properties of the alloy 33 are tested in comparison to those of reference materials in high chloride containing solutions (1M NaCl, artificial and modified sea water, 10% FeCl₃ · 6H₂O; 500 g/l CaCl₂ ). Pitting potentials and potentials of repassivation of pitting, critical temperatures of localized corrosion (FeCl₃-test, CaCl₂-test, artificial sea water), potentials of repassivation of crevice corrosion as well as depassivation pH values of crevice corrosion following Crolet have been determined. The results confirm that the localized corrosion behaviour of the alloy 33 corresponds to its PREN. With regard to pitting corrosion alloy 33 is comparable with the special stainless steel alloy 31 (mater. No. 1.4562), with regard to crevice corrosion it is comparable with alloy 926 (German mater. No. 1.4529).     

Pitting and crevice corrosion of superaustenitic stainless steels

Superaustenites are mainly used in offshore applications, oil production and chemical industry. Most important types of localised corrosion of these steels are pitting and crevice corrosion. Investigated materials were N08028, S31254 and three modified alloys. Chromium content of investigated alloys varied between 20 and 27%, molybdenum between 3.2 and 6.0%, nitrogen between 0.1 and 0.36% and copper between 0 and 1.1%. For means of comparison stainless steel AISI 316L has been included in the study. Pitting and crevice corrosion of these highly corrosion resistant steels has been investigated by use of standardized tests. Critical pitting temperature and critical crevice temperatures were determined according to ASTM G 48, Methods C and D, respectively. Electrochemical measurements for determination of pitting potentials were done according to ASTM G 61 as well as for determination of critical pitting temperatures according to ASTM G 150. Results are presented as function of MARC (Measure of alloying for resistance to corrosion) defined by Speidel since linear correlation coefficients were higher when compared to conventional PREN. Results obtained by different testing methods must not be compared directly. Every test however is sensitive to microstructural defects like precipitations and segregations that decrease corrosion resistance. The higher alloyed a material is, the higher is its tendency to form microstructural defects, and the more difficult is it to reach its theoretical corrosion resistance at given chemical composition.     

Localized corrosion properties of low interstitial ferritic,high purity austenitic and high chromium duplex stainless steels

Within the framework of a research aimed at characterizing the behaviour of new materials to pitting and crevice corrosion, an investigation has been made, using electrochemical techniques, of the following materials: ELI ferritic stainless steels (18 Cr-2 Mo-Ti; 21 Cr-3 Mo-Ti; 26 Cr-1 Mo); high chromium duplex stainless steel (Z 5 CNDU 21-08) and high chromium-nickel austenitic stainless steel (Z 2 CNDU 25-20); commercial austenitic stainless steels (AISI 304 L and 316 L) and laboratory heats of austenitic stainless steels with low contents of interstitials (LTM/18 Cr- 12 Ni, LTM/16 Cr- 14 Ni-2 Mo). It was possible to graduate a scale of resistance to pitting and crevice corrosion in neutral chloride solutions at 40 C; in particular the two experimental austenitic stainless steels LTM/18 Cr- 12 Ni and LTM/16 Cr- 14 Ni-2 Mo are at the same level as the AISI 316 L and 18 Cr-2 Mo-Ti, respectively. An occluded cell was developed and used for determining the critical potential for crevice corrosion (E_{localized corrosion}). For the steels under investigation E_{localized corrosion} is less noble than E_pitting especially for ELI ferritic 18 Cr-2 Mo-Ti and 21 Cr–3 Mo-Ti.     

Corrosion behaviour of alloy 31 – UNS N08031 – under conditions of oil & gas production

The corrosion behaviour of alloy 31 (UNS N08031‐31Ni – 27Cr – 6.5Mo – 1.2Cu – 0.2N – bal. Fe) was tested in laboratory and field tests in seawater with and without additions of CO₂ and/or H₂S in slow strain rate tests, and in SSC (Sulphide Stress Corrosion) tests according to NACE MR0175. The results demonstrate a high resistance of alloy 31 to localised corrosion. Due to the high chromium and molybdenum concentration, its resistance to pitting and crevice corrosion in chloride‐contaminated seawater is significantly higher than that of alloy 28 and alloy 825 and it equals that of typical nickel base alloys like alloy 625. Alloy 31 is not sensitive to chloride‐induced stress corrosion cracking, either with or without H₂S, or sulphide stress cracking. Alloy 31 is approved for sour gas applications up to LEVEL VI in NACE MR 0175. The combination of properties makes alloy 31 an attractive choice for components in oil and gas production including wirelines, umbilicals, tubing, piping and topside application.     

Initiation and repassivation of crevice corrosion of type 444 stainless steel in chloride solution

Effects of chloride ion concentrations, solution temperature, and crevice-forming materials on the crevice corrosion of type 444 stainless steel were investigated using a potentiostatic method. Critical crevice potential (E_crev) and repassivation potential (E_r) of the creviced alloy decreased with an increase in chloride concentration [Cl⁻], satisfying the logarithmic relationship between E and [Cl⁻]. In addition, E_crev and E_r of the alloy with silicone crevice former were measured to be higher than those of the alloy with an EPDM (Ethylene Propylene Diene Monomer) crevice former, suggesting that silicone was more effective in preventing water from penetrating crevices between a stainless steel sheet and the crevice former. In electrochemical current transient measurements with an applied potential, the intensity of current transients corresponding to the initiation of metastable pits increased abruptly near the E_r of the alloy, indicating that the stability of crevice corrosion is associated with the initiation of metastable pits.     

Elektrochemische Methode zur Prüfung des Korrosionsverhaltens austenitischer CrNiMo-Stähle im Schweißnahtbereich

Electrochemical method for testing of the corrosion behaviour of austenitic CrNiMo-stainless steel weldments Austenitic CrNiMo stainless steel welds can be more susceptible to pitting and crevice corrosion than the base material owing to segregation and precipitation in the heat affected zone, in the high temperature zone and in the welded zone. Suitable test methods are needed to optimize welding technology. Comparison of potential curves (DIN 50919), comparison of critical pitting corrosion temperatures in FeCl₃- or other model solutions, visual estimation after longterm corrosion tests with or without electrochemical load are discussed. A small measurement cell heated from the rear with circulating and temperature controlled electrolyte is shown. With its help separate areas of a weld can be electrochemically investigated. Welds of the materials X 2 CrNiMoN 17 12 2 (1.4406), X 2 CrNiMoN 17 13 5 (1.4439) and X 1 NiCrMoCu 31 27 4 (1.4563) are tested in acidic NaCl solutions in the range of 25 to 75°C. Critical pitting corrosion potentials are obtained for base materials, heat affected zone and weld material. An influence of the welding energy is probable, but cannot be proved in this case without statistical certainty.     

Temperature as a pitting and crevice corrosion criterion in the FeCl3 test

The FeCl₃ test is applied to an increasing extent for examining the resistance to pitting and crevice corrosion. Two methods having proved their value are described, the chemical properties of the FeCl₃ solution with regard to hydrolysis, pH and redox potential behaviour at various test temperatures are set forth and finally numerous results of the application of this test to high-alloy stainless steels and nickel alloys are presented. These results have been used to establish, be means of multiple regression, two empirical equations that allow to estimate rather accurately the critical pitting and crevice corrosion temperatures (CPT, CCT) from the contents of the decisive alloying constituents. These temperatures vary by about 2.5°C in the CPT test and by approx. 10°C in the CCT test, which can be reduced, however, by extending the test period beyond 24 hours. This is due to the fact that corrosion potentials in a 10% FeCl₃ · 6H₂O solution take a long time to stabilize. The variation of the critical crevice temperature can be further reduced by pressing the crevice blocks at a higher torque to the specimen. Another section particularly deals with the application of the CPT test for determining the influence of the matrix on the resistance to local corrosion. Consequently, the CPT test lends itself excellently to the examination of welds and as a quality control. Finally, CPT test results are compared with pitting data determined electro-chemically in artificial seawater. This shows that the ranking order with regard to corrosion resistance is identical, although media and processes differ considerably from each other.     

Die Lochkorrosionsbegrenzung bei hochlegierten Sonderedelstählen und NiCrMo‐Legierungen in Chloridlösung – Einflußfaktoren und Ausmaß

Limit of pitting corrosion at high‐alloyed special steels and NiCrMo alloys in chloride solution The phenomenon of the limit of pitting corrosion in direction to positive potentials is studied by potentiokinetic polarization after a jump in the transpassive range and by potentiostatic tests at technical wrought materials and at model alloys of the systems NiCrMo and NiMo in CaCl₂ solution in the concentration range 1 to 9 mol/l chloride at pH‐values of 1 to 9 at temperatures of 30 to 110°C. Surface‐analytical investigations gives in connection with knowledges from anodic polarization studies directions to the mechanism of the limit of pitting corrosion. Ranges of the limit of pitting corrosion are obtained at materials with a Mo content above 6.5% and contents of chloride of the media above 2 mol/l chloride. Increasing temperatures, increasing contents of chloride and sulfate shift the potential of the limit of pitting corrosion being always above 0.2 V (SCE) at potentiostatic determination to noble direction. There are indications that the mechanisms of limit of pitting corrosion is resulting from an inactivation of pitting nuclei by the formation of hardly soluble molybdenum chlorides in the potential range of limit of pitting corrosion.     

Influence of activated carbon on crevice corrosion in adsorption tower of advanced liquid processing system

ABSTRACT

The adsorption tower made of type 316L stainless steel (SS) in Multi-nuclide Removal Equipment (Advanced Liquid Processing System) which uses Ag-impregnated activated carbon (Ag AC) as an adsorbent experienced crevice corrosion. The influence of Ag AC on the crevice corrosion susceptibility and E_sp of 316L SS was investigated by performing electrochemical experiments. Crevice corrosion was observed in the specimen in contact with the Ag AC. On the other hand, there was no crevice corrosion without the Ag AC in both pH 7.4 and pH 12 solutions. Clear ennoblement of spontaneous potential (E_sp) by in contact with activated carbon was observed and that was clearly higher than the repassivation potential for crevice corrosion (E_R,CREV). Thus, the presence of the AC notably increased E_sp of 316L SS and this resulted in increased crevice corrosion susceptibility by the galvanic effect.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

Monitoring corrosion rates and localised corrosion in low conductivity water

Monitoring of low corrosion rates and localised corrosion in a media with low conductivity is a challenge. In municipal district heating, quality control may be improved by implementing on-line corrosion monitoring if a suitable technique can be identified to measure both uniform and localised corrosion. Electrochemical techniques (LPR, EIS, crevice corrosion current) as well as direct measurement techniques (high-sensitive electrical resistance, weight loss) have been applied in operating plants. Changes in the corrosion processes are best monitored in non-aggressive, low conductivity media with sensitive electrical resistance technique and crevice corrosion current measurements.     

Corrosion resistant coatings based on synergistic effects of alkaline etching and molybdate treatment for AZ31D magnesium alloy

Eco-friendly, non-toxic protective coatings deposited from molybdate–based solutions have been developed as undercoat thin-films (for subsequent organic top coats) for AZ31D magnesium alloy. Direct treatment of Mg AZ31D substrates with molybdate–based solutions has no significant effect on the overall surface resistance (charge transfer resistance). Alkaline etching of Mg AZ31D surfaces using KOH solution prior to molybdate conversion coating showed significant enhancement in the corrosion resistances The optimum conditions of alkaline etching and molybdate treatment steps have been determined. The total surface resistance was improved from 2.1?×?10³ Ω.cm² (for as-polished AZ31D) to be 3.2?×?10³ Ω.cm² for the alkaline etched samples followed by 10?g?L^?1 molybdate treatment. The resistance to localised corrosion (pitting and crevice) improved significantly after applying the alkaline etching step. Molybdate–based coatings formed on Mg AZ31D exhibited a network of flower-like and needle-like protective molybdenum oxide structures which are belived to be responsible for the improvement in the pitting and crevice corrosion resistance. They impede the corrosive media from reaching the bare metal and hence improve the pitting and crevice corrosion resistances. This simple eco-friendly, low-toxicity pretreatment approach seems very promising and effective for improving the corrosion resistance of magnesium alloys in chloride containing solutions.     

Effects of pH and chloride concentration on pitting corrosion of AA6061 aluminum alloy

Effects of pH solution and chloride (Cl⁻) ion concentration on the corrosion behaviour of alloy AA6061 immersed in aqueous solutions of NaCl have been investigated using measurements of weight loss, potentiodynamic polarisation, linear polarisation, cyclic polarisation experiment combined with open circuit potential transient technique and optical or scanning electron microscopy.The corrosion behaviour of the AA6061 aluminum alloy was found to be dependant on the pH and chloride concentration [NaCl] of solution. In acidic or slightly neutral solutions, general and pitting corrosion occurred simultaneously. In contrast, exposure to alkaline solutions results in general corrosion. Experience revealed that the alloy AA6061 was susceptible to pitting corrosion in all chloride solution of concentration ranging between 0.003 wt% and 5.5 wt% NaCl and an increase in the chloride concentration slightly shifted both the pitting E_pit and corrosion E_cor potentials to more active values. In function of the conditions of treatment, the sheets of the alloy AA6061 undergo two types of localised corrosion process, leading to the formation of hemispherical and crystallographic pits.Polarisation resistance measurements in acidic (pH = 2) and alkaline chloride solutions (pH = 12) which are in good agreement with those of weight loss, show that the corrosion kinetic is minimised in slightly neutral solutions (pH = 6).     

The nature of crevice corrosion of aluminum in chloride solutions

To study crevice corrosion of pure aluminum, polished specimens partly covered with a glass foil were polarized potentiostatically in 1 N NaCl-solution at potentials negative to the critical potential for stable pitting (pitting potential). For comparison, non-crevice experiments were performed on polycrystalline and singlecrystalline material in neutral as well as acidified 1 N NaCl-solution and in AlCl₃-solutions. Corrosion morphology was examined by scanning electron microscopy. In current-time plots recorded during experiments on crevice corrosion, both an incubation and a propagation stage are discernible. If experiments were interrupted during the induction period, micropits were found inside the crevice. This unstable micropitting is detectable down to 0.30 V below the pitting potential. In contrast, during crevice corrosion propagation, the aluminum surface undergoes general attack. In a range of 0.2 V below the pitting potential, dimpled surfaces are produced. At more negative potentials, metal dissolution occurs crystallographically oriented. An identical behaviour was detected on unshielded samples polarized in the same potential range in both 1 N AlCl₃- and acidified 1 N NaCl-solution. Hence, the build-up of an acidic electrolyte is considered the sufficient requirement for crevice corrosion initiation.     

Corrosion behaviour of Lotus-type porous high nitrogen nickel-free stainless steels

Corrosion behaviour of three austenitic Lotus-type porous high nitrogen Ni-free stainless steels exposed to an acidic chloride solution has been investigated by electrochemical tests and weight loss measurements. Polarization resistance indicates that the corrosion rate of Lotus-type porous high nitrogen Ni-free stainless steels is an order of magnitude lower than that of Lotus-type porous 316L stainless steel in acidic environment. The localised corrosion resistance of the investigated high nitrogen Ni-free stainless steels, measured as pitting potential, E_b, also resulted to be higher than that of type 316L stainless steel. The influences of porous structure, surface finish and nitrogen addition on the corrosion behaviour were discussed.     

Corrosion resistance and mechanical properties of nickel-bearing ferritic stainless steels

Additions of nickel to ferritic steels containing 25–28% Cr and 2–4% Mo increased the impact toguhness especially when more than 2% Ni was present. The effect of nickel content increased up to 4% Ni, the largest addition studied. Steels stabilized with niobium had lower transition temperatures then did corresponding steels stablizied with titanium. Steels containing 4% Ni required annealing at 1050 C to avoid intermetalic compounds. It was also noted that nickel reduced the upper shelf energy in the Charpy impact test and eliminated a sharp transition from ductile to brittle behaviour. No definite effect of nickel on pitting potential was pound but steels in the series 25Cr-3.5 Mo-Ni-Ti consistenly had more noble pitting potentials and greater resistance to crevice corrosion than the 28 Cr-2Mo-Ni-Ti steels. Nickel contents of 1 or 2% tended to improve crevice corrosion resistance while larger nickel contents were somewhat ditrimental. Nickel strongly reduced critical current densities for passivity both in l N H₂SO₄ and in l N HCL and yielded corresponding increases in resistance to corrosion by these acids. Although 1% Ni or more caused the annealed steels to be susceptible to stress corrosion cracking in MgCl₂ boiling at 140 C, while the as-Welded steels containing 4% Ni did not crack in boiling 25% Nacl at pH 1.     

The effect of annealing on the corrosion behaviour of 444 stainless steel for drinking water applications

In this study, the corrosion behaviour of annealed and not annealed AISI 444 ferritic stainless steel in tap water with and without addition of selected concentrations of chloride ions was investigated. Cyclic potentiodynamic macro (large area) and micro (small area) polarization measurements (CPP), salt spray test, SEM and EDS analysis were employed to evaluate the pitting and crevice corrosion susceptibility of annealed and not annealed AISI 444. The results obtained indicate that annealing does not improve the resistance to pitting and crevice corrosion. Moreover, micro CPP indicates local susceptibility to pitting on both annealed and not annealed materials; such susceptibility was not evident from macropolarization tests.     

Korrosionsverhalten hochchromhaltiger,ferritischer, chemisch beständiger Stähle

Corrosion behaviour of high chromium ferritic stainless steels Ferritic steels developed for seawater desalination and containing 20 to 28% chromium, up to 5% Mo and additions of nickel and copper have been tested with respect to their corrosion behaviour, in particular in chloride containing media. The materials in the sensibilized state were tested for inter-crystalline corrosion susceptibility in the Strauß-, Streicher-, nitric acid hydrofluoric acid- and Huey-Tests. No intercrystalline corrosion was encountered in the case of the steels with 28% Cr and 2% Mo. The resistance to pitting was assessed on the basis of rupture potentials determined by potentiokinetic tests. The resistance of the steels with 20% Cr and 5% Mo or 28% Cr and 2% Mo is superior to that of the molybdenum containing austenitic types. Addition of nickel yields a significant increase in crevice corrosion resistance; the same applies to resistance in sulfuric acid. In boiling seawater all the materials tested are resistant to stress corrosion cracking. No sign of any type of corrosion was found on nickel containing steels after about 6000 hours exposure to boiling 50% seawater brine even under salt deposits.     

The stability of localized corrosion

Chloride pitting of iron group metals at noble potentials proceeds in the polishing state dissolution, provided that metal chloride in the pit solution is maintained above a critical concentration. It ceases to progress by pit repassivation if the pit is smaller than a critical size, or transforms into the active state pitting if the pit size is greater. The boundary potential between the polishing state and the active state pitting may be represented by the passivation-depassivation potential in the pit solution of the critical chloride concentration. Crevice corrosion is characterized by the crevice protection potential, at which the hydrogen ion concentration in the crevice solution is equivalent to pH_pd—the passivation-depassivation pH of the crevice metal. It continues to corrode at more noble potentials than the protection potential, where the crevice solution is more acidic than pH_pd, but is inhibited in the less acidic crevice solution at less noble potentials.