Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels

Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels （HNSS） were investigated by electrochemical and immersion testing methods in chloride solution, respectively. The chemical constitution and composition in the depth of passive films formed on HNSS were analyzed by X-ray photoelectron spectrum （XPS）. HNSS has excellent pitting and crevice corrosion resistance compared to 316L stainless steel. With increasing the nitrogen content in steels, pitting potentials and critical pitting temperature （CPT） increase, and the maximum, average pit depths and average weight loss decrease. The CPT of HNSS is correlated with the alloying element content through the measure of alloying for resistance to corrosion （MARC）. The MARC can be expressed as an equation of CPT=2.55MARC-29. XPS results show that HNSS exhibiting excellent corrosion resistance is attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation, and the synergistic effects of molybdenum and nitrogen.

Numerical Simulation for Crevice Corrosion of 304 Stainless Steel in Sodium Chloride Solution

The authors presented a mechanistic model describing the chemical reactions within a corroded thin, narrow crevice. In the mathematical model, a two-dimensional steady-state was used to predict the crevice pH profile by taking into account dissolved oxygen and hydrogen ions within the crevice. It consists of six parallel electrochemical reactions： multi anodic reactions（Fe, Cr, Ni dissolution reactions） and three cathodic reactions（the oxygen reduction, the hydrogen reaction and water dissociation）. Current density distribution and oxygen concentration distribution were determined to be corresponding to the evolution of potential distribution within the crevice. The contribution of each metal reaction to the overall corrosion process was in proportion to the mole fraction, and the simulation pro vided a good agreement with published experimental results for the crevice corrosion of stainless steel in sodium chloride solution.

Corrosion products of reverse crevice corrosion of copper

Open-circuit potential measurements and Raman spectroscopy were used to investigate the reverse crevice corrosion phenomenon and its corresponding corrosion products. With the aid of these techniques, the existence of reverse-crevice corrosion in copper was verified, i.e., while the surface of a crevice was corrosion free, the outside surface of the copper was attacked. The processes associated with this phe-nomenon were classified into three phases, and different compositions of the corrosion products were determined. Raman spectra showed that copper and Cu2O were found in the crevice, while CuO, Cu2O, and CuCl2 were the corrosion products on the bold surface. Based on these findings, a hypothesis relating to the three phases of reverse crevice corrosion has been proposed.

Development of a Test Program for Assessing the SCC, Pitting and Crevice Corrosion Behaviour of Selected Corrosion Resistant Alloy Liner Materials for Clad Pipe for Offshore Applications

The development of an offshore gas field involves production of oil and gas from subsea wells. Design considerations for a particular subsea system have identified the maximum temperature for operations which is greater than 130 ~C. Consequently, for large diameter flowlines, this precludes the use of duplex stainless steels as CRA＇s （corrosion resistant alloys） for service under the expected operating conditions. Attention has therefore focused on alternative CRA＇s such as austenitic stainless steels and Ni based alloys as mechanically clad or lined clad pipe for carbon steel subsea gas flowlines. One design concern is the recognized temperature limit for CRA materials, such as 316L stainless steel and similar alloys in production fluids is taken to be 120 ~C. This then raises concerns surrounding their suitability for providing sufficient corrosion resistance/stress corrosion cracking resistance in gas environments containing COz and H2S at temperatures higher than 130 ~C. It was recommended that specific testing should be completed at temperatures greater than 133 ~C to establish that candidate materials are still corrosion resistant. The focus of this study was to develop, implement and assess a testing program that would predict the suitability of a series of CPA＇s alloys for use in mechanical clad/lined subsea gas flowlines, with respect to pitting corrosion, crevice corrosion and stress corrosion cracking. This paper will present the rationale adopted for this testing program to simulate the stringent operating conditions, the results from these findings and the overall assessment/integrity of the candidate alloys selected.

Failure analysis of crevice corrosion on 304 stainless steel tube heat exchanger

The cause of the premature failure of 304 stainless steel tube heat exchanger was investigated.The unique skeleton structure inside the leakage point reveals that this is a new damage mechanism caused by a δ+γ two-phase structure and crevice corrosion.The three-dimensional structure of the leakage point was demonstrated using X-ray diffraction topography.The results of scanning electron microscope examination show the microstructure of the weld to be columnar and dendritic.It is found by electron probe microscope analysis and transmission electron microscopy that columnar dendrites consisted of γ-dendrite and an amount of δ-ferrite phases at the dendrite trunk.Simulated corrosion test results confirmed that the corrosion medium was the chloride ion.Crevice corrosion of chloride ions occurred at weld defects on the inner wall thus forming a concentration cell.Grains of columnar dendrites were then corroded by chloride ions and δ-ferrite phases on the grain boundaries were retained,which formed the particular skeleton corrosion structure.As a result,it led to leakage when the corrosion of weld occurred from the inner wall to the outer wall.

Effects of 405 stainless steel on crevice corrosion behavior of Alloy 690 in high-temperature pure water

Effects of 405 stainless steel(405 SS) on crevice corrosion behavior of Alloy 690 in high-temperature pure water were investigated.Results revealed that the corrosion rate of Alloy 690 was low within the crevice.It was likely attributed to the fact that a Cr-rich inner oxide film and a Ni-rich layer beneath this oxide film formed upon Alloy 690,inhibiting the diffu sion of oxygen towards the oxide/matrix interface.Moreover,the Fe2+ions dissolved from 405 SS consumed most of oxygen,leading to less oxygen participating in the oxidation of Alloy 690.In addition,it was found that Fe concentration continuously decreased from the surface of the inner oxide film to the oxide/matrix interface of Alloy 690 within the crevice,which was probably due to the diffusion of Fe2+ions dissolved from 405 SS into the inner oxide film.

Effects of thiosulfate and dissolved oxygen on crevice corrosion of Alloy 690 in high-temperature chloride solution

The crevice corrosion behavior of Alloy 690 was investigated in high-temperature chloride solution containing different concentrations of thiosulfate(S_(2)O_(3)^(2-)) and dissolved oxygen(DO). The S2O32- inhibited corrosion of Alloy 690 through consuming oxygen in aerated chloride solution. In deaerated chloride solution,the S_(2)O_(3)^(2-) was reduced to S° and S^(2-) and subsequently incorporated into the oxide films, which promoted the pitting corrosion at locations of both TiN inclusions and matrix inside the crevice during the crevice corrosion as well as influenced the composition and thickness of the oxide films. The effect mechanisms of S_(2)O_(3)^(2-) and DO on crevice corrosion in high-temperature chloride solution are discussed.

A multiphysics model for studying transient crevice corrosion of stainless steel

The transient crevice corrosion behavior of 304 stainless steel in NaCl solution has been investigated by a multiphysics coupling model.The model considers local electrochemical reactions,transport of diffe rent species,and homogeneous reactions.The moving mesh method is used to obtain the geometrical change of the crevice wall with time due to corrosion.The level set method is employed to quantitatively describe the influence of the precipitation process on electrochemical reactions.The transient crevice corrosion morphology,potential and current distributions,and pH and chloride ion concentration distributions are obtained by simulation.The effect of crevice geometry factors on the corrosion process is also discussed.The simulation results are in good agreement with the experiments,showing that the model has high reliability.

Crevice Corrosion Performance of 436 Ferritic Stainless Steel Studied by Different Electrochemical Techniques in Sodium Chloride Solutions with Sulfate Addition

The crevice corrosion behaviors of 436 stainless steels in chloride-containing solutions with sulfate addition were studied using potentiodynamic, galvanostatic and repassivation potential measurements. The results of these electrochemical tests were compared and discussed. Galvanostatic test was proved to be the most powerful technique in detecting the crevice corrosion of 436 stainless steels, while the repassivation potential measurement was the most time-saving method in this study. Sulfate ions have inhibited the crevice corrosion of 436 stainless steels in chloride-containing solution, which may result from the effects of competitive adsorption and the IR drop mechanism.

A discussion on evaluation criteria for crevice corrosion of various stainless steels

In this study,crevice corrosion performances of a newly developed LDSS 2002 and three commercial stainless steels(AISI 304,AISI 316L and DSS 2205)were investigated and discussed.Crevice repassivation potential(ER,CREV),which was measured by the potentiodynamic-galvanostatic-potentiodynamic(PDGS-PD)test,was applicable to crevice corrosion evaluation of 304 and 316 L stainless steels.However,much lower(ER,CREV values were obtained for DSS 2205 and LDSS 2002.These abnormal(ER,CREV values for duplex stainless steels may be related to the selective attack of the less corrosion-resistant phase,the lower corrosion potential in the crevice-like solution,and more crevice corrosion sites in the PD-GS-PD test.A critical chloride concentration of crevice corrosion(CCCCREV)measurement was introduced for crevice corrosion evaluation of various stainless steels.The derived CCCCREVwas proved to be a valid criterion for crevice corrosion evaluation of both the austenitic and duplex stainless steels.An order of crevice corrosion resistance of AISI 304≈LDSS 2002<AISI 316 L<DSS 2205 was suggested,which agreed well with the orders of pitting resistance equivalent number and critical crevice index of the less corrosion-resistant phase in each material.

Crevice Corrosion of Several Supper Stainless Steels in the Simulated LT-MED Environment

Susceptibility and morphological characteristics of crevice corrosion for SS316, SS904L, SS254sMo and SS2507 in thesimulated low-temperature multi-effect distillation environment were investigated by cyclic polarization test, scanningelectron microscope and laser microscope. The results show that the crevice corrosion resistance of four kinds of stainlesssteel is ranked as SS254sMo ≈ SS2507 〉 SS316 〉 SS904L. There are ＂cover＂ structures over the edge of active crevicecorrosion regions of SS904L, SS254sMo and SS2507, but SS316 is an exception. Galvanic corrosion characteristicsappeared in the crevice of duplex supper stainless steel SS2507.

High throughput screening of localised and general corrosion in type 2205 duplex stainless steel at ambient temperature

Bipolar electrochemistry is used to produce a linear potential gradient across a bipolar electrode(BPE),providing direct access to the anodic and cathodic reactions under a wide range of applied potentials.The occurrence of pitting corrosion,crevice corrosion,and general corrosion on type 2205 duplex stainless steel(DSS 2205)BPE has been observed at room temperature.The critical pit depth of 10-20μm with a55%-75% probability of pits developing into stable pits at potential from+0.9 to+1.2 V vs.OCP(open circuit potential)are measured.All pit nucleation sites are either within ferritic grains or at the interface between austenite and ferrite.The critical conditions for pitting and crevice corrosion are discussed with Epit(critical pitting potential)and Ecre(critical crevice potential)decreasing from 0.87 and 0.80 V vs.OCP after150 s of exposure to 0.84 and 0.76 V vs.OCP after 900 s of exposure,respectively.Pit growth kinetics under different applied bipolar potentials and exposure times have been obtained.The ferrite is shown to be more susceptible to general dissolution.

In vitro crevice corrosion of biodegradable magnesium in different solutions

Magnesium(Mg)is a promising biomedical metal because of its biodegradability.The crevice between tissue and Mg implant can not be neglected in some implantation sites due to inducing crevice corrosion of Mg.In this paper,a new single mold was designed to build the in vitro experimental setup and four kinds of solutions,i.e.the deionized water(DW),the 0.9 wt.%sodium chloride solution(NaCl),the phosphate buffer saline(PBS)and the modified simulated body fluid(m-SBF)were used to explore necessary factors of crevice corrosion in Mg.It was observed that crevice corrosion in Mg sheets would occur in NaCl and PBS solution under 0.2,0.5 and 0.8 mm crevice thickness.And it was found that there were two necessary factors,i.e.chloride ion and crevice dimension,in crevice corrosion.For the high-purity Mg cannulated screws,crevice corrosion could occur inside tunnel when immersed in PBS.

Crevice Corrosion of K60 in Dry Desertification Saline Soil

Corrosion products of K60crevice samples which were buried in Ordos area were analyzed by scanning electron microscopy and X-ray diffraction.The result showed that,the corrosion potential decreased differently in the crevice and a maximum negative existed at the bottom side;with pH value reduced,the dissolved oxygen was nearly exhausted in the crevice.When the opening side of the coupons was coupled with each other,the crevice corrosion in the crevice might be accelerated and the pH value gradually reduced;however,the oxygen was not completely exhausted.The unbalanced current density between anode and cathode was the main cause of blocked self-catalysis effect in the crevice,and was also the main cause of corrosion of alloys.Lumpy corrosion happened on the surface of metal,and rusty layer was composed of Fe3O4,Fe2O3,FeO,FeOOH and FeS which distributed in the opening of crevice.There were the reddish-brown corrosion products and white matter which distributed in the form of filiform and spotted in the center part of crevice.The analysis showed that the white matter might include oxide of Fe and NaHCO3.

Localized Corrosion Behavior of 6% Mo Super Austenitic & 316L Stainless Steels in Low pH 3% NaCl Solution

Electrochemical techniques were applied to study the crevice corrosion resistance of two types of stainless steel alloys namely, conventional 316L and 6% Mo super austenitic in acidified 3% NaCl solution at room temperature.Potentiodynamic results showed that 6% Mo alloy possessed a remarkable resistance to crevice corrosion compared with 316L alloy when they are tested in the same solution. The breakdown potential at which passivity broke down for 316L alloy was 0.00 mV （SCE）. The corresponding value for 6% Mo alloy could not reach up to the potential value of 700 mV （SCE）. 316L alloy suffered extremely from crevice corrosion at room temperature （about 25℃）, which indicates that the critical crevice corrosion temperature, below which crevice corrosion does not occur, was lower than the test temperature. For 6% Mo alloy, the critical crevice corrosion temperature was higher than the testing temperature. Electrochemical parameters indicated that 6% Mo alloy exhibited higher crevice corrosion resistance than 316L alloy.

Effects of solution environments under disbonded coatings on the corrosion behaviors of X70 pipeline steel in acidic soils

A rectangle crevice assembly was used to study the effects of cathodic protection （CP） potential, crevice thickness, holiday size, bubbling CO2, and surface condition on the chemical and electrochemical environment of the local solution under disbonded coatings. It is found that the cathodic protection removes dissolved oxygen from the crevice and thus shifts the solution to a more al- kaline state. Furthermore, the potential of the steel reaches the protected potential range. The available protection distance increases with the negative applying potential. The steady potential and pH distribution are easily achieved, but the polarization degree is not satisfied within the thinner crevice. The difference in the solution environment is found to correlate to the holiday size. The smaller the holiday, the smaller the difference is. The presence of CO2 inhibits the formation of an alkaline environment. It is also found that the rust layer dramatically decreases the polarization rate in the crevice.

Electrochemical corrosion behavior of carbon steel with bulk coating holidays

With epoxy coal tar as the coating material, the electrochemical corrosion behavior of Q235 with different kinds of bulk coating holidays has been investigated with EIS （Electrochemical Impedance Spectroscopy） in a 3.5vo1% NaCI aqueous solution. The area ratio of bulk coating holiday to total coating area of steel is 4.91%. The experimental results showed that at free corrosion potential, the corrosion of carbon steel with disbonded coating holiday is heavier than that with broken holiday and disbonded ＆ broken holiday with time; Moreover, the effectiveness of Cathodic Protection （CP） of carbon steel with broken holiday is better than that with disbonded holiday and disbonded ＆ broken holiday on CP potential -850 mV （vs CSE）. Further analysis indicated that the two main reasons for corrosion are electrolyte solution slowly penetrating the coating, and crevice corrosion at steel/coating interface near holidays. The ratio of impedance amplitude （Z） of different frequency to minimum frequency is defined as K value. The change rate of K with frequency is related to the type of coating holiday.

Development of a two-dimensional bipolar electrochemistry technique for high throughput corrosion screening

Bipolar electrochemistry allows testing and analysing the crevice corrosion,pitting corrosion,passivation,general corrosion,and cathodic deposition reactions on one sample after a single experiment.A novel two-dimensional bipolar electrochemistry setup is designed using two orthogonal feeder electrode arrangements,allowing corrosion screening tests across a far wider potential range with a smooth potential gradient to be assessed.This two-dimensional bipolar electrochemistry setup was applied here to simultaneously measure for the simultaneous measurement of the nucleation and propagation of pitting and crevice corrosion under a broad range of applied potential on type 420 stainless steel,which has a very short localised corrosion induction time.It reduces the error from corrosion induction to corrosion competition,and all pits and crevice corrosion have no lacy cover.Results show crevice corrosion can gain current density and easier to support its nucleation and propagation at different potential regions more easily than pitting corrosion.

Corrosion behavior of 13Cr stainless steel under stress and crevice in high pressure CO_(2)/O_(2)environment

In this work,through the self-developed setup for in situ electrochemical tests,the corrosion behavior of 13 Cr stainless steel under the combined effect of stress and crevice in high pressure CO_(2)/O_(2)environment was investigated.The results show that 13 Cr stainless steel presents a self-passivation state.Under the action of stress,the anodic dissolution process of steel inside crevice is expedited.There is a galvanic effect between the stressed steel inside crevice and the unstressed steel outside crevice.The applied stress reduces the induction stage of crevice corrosion and induces a larger galvanic current,i.e.,the applied stress promotes the development of crevice corrosion.Meanwhile,adding 0.1 MPa O_(2)is conducive to forming a more stable passive film but causes a greater galvanic effect.