Stress corrosion cracking tests using double-cantilever-beam specimens

Simulation of Crevice and Pitting Corrosion Using the Orthogonal Collocation Method

Corrosion of brass

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

In this overview passivation mechanisms of high nitrogen steels (HNS) are decribed first and then the various corrosion properties related to general corrosion, pitting corrosion, crevice corrosion, intergranular corrosion (sensitization), stress corrosion cracking and hydrogen embrittlement are summarized based on the present knowledge. The effects of nitrogen on pitting corrosion of stainless steels have been studied most systematically and a number of different mechanisms have been proposed for the effect of nitrogen on pitting and crevice corrosion related to observed synergistic interactions with Mo in the alloy. There is a fairly consistent agreement that N additions improve the pitting corrosion resistance of austenitic and duplex stainless steels by increasing the pitting potential or temperature in aqueous chloride solutions or by decreasing the weight loss in standard FeCl 3 test. There is much less results on the effects of nitrogen alloying on active dissolution or general corrosion, and almost no systematic studies on the effects of N content on environment sensitive cracking of these steels.

Chemical pitting corrosion tests in neutral 1.0 M NaCl solution containing 0.5 M K3Fe(CN)6 and measurements of pitting potentials Vc′ and Vcre′ for stainless steel open surface and creviced specimens in 0.5 M NaCl solution were conducted to study the resistance to pitting and crevice corrosion of 18 Cr-14 Ni stainless steels to which the alloying elements Mo, Si and N had been added. The following results were obtained. (1) Mo and N increased the resistance to both pitting and crevice corrosion, whereas Si increased only the pitting resistance and had no effect on the crevice corrosion tendency. The beneficial effects of Si and N were found to be pronounced especially when 1∼3%Mo had been alloyed together with them. (2) In many cases, the pitting potentials of creviced specimens Vcre′ were found to be 0.3∼0.4 V more negative than those of open surface specimens. However, in the case of 18 Cr-14 Ni-3.5 Si-0.2 N stainless steel, Vcre′ was about 0.9 V more negative than Vc′, indicating a greater susceptibility to crevice corrosion. For practical purposes of assessing the susceptibility and/or resistance of an alloy to localized corrosion, it is essential that the pitting potential Vcre′ of the creviced specimen as well as that of the open surface, Vc′, be measured.

The resistance of X5CrNi18-10 stainless steel to pitting corrosion in a solution containing chlorides and sulphates was tested using the potentiodynamic polarization method. The obtained results show that the stainless steel is significantly resistant to pit formation, but it is susceptible to pit growth and crevice corrosion. Pits formed at the corrosion potential grow continuously. Statistical analysis of the results obtained during pitting corrosion testing was performed. It can be assumed with a probability of 95% that values of indicators of resistance to pit formation (the pitting potential Epit, the metastable pitting potential Empit and the difference Epit-Ekor) will be within the range of several percents. Values of indicators of resistance to pit growth (the amount of charge required for the pit growth q, the protective potential Eprot and the difference Epit-Eprot) will be within the broader range. In addition, the appearance of pits on the surface of the stainless steel, as well as the appearance of the pits bottom, were analyzed. It was shown that the structure of the stainless steel tested was not sensitized to pitting and intergranular corrosion, which means that the stainless steel was not previously thermally treated.

Oil and gas pipelines manufactured from API‐5L Grade X65 steel are generally subjected to cyclic loading and their internal surfaces are frequently exposed to corrosive sour fluids. Exposure of pipelines to these environments often leads to localized corrosion (pitting) and decreased fatigue life. Corrosion pits are geometrical discontinuities that may promote fatigue cracking by acting as stress raisers. In order to optimize asset inspection and repair scheduling, it is important to understand the fatigue behavior of X65 steel and in particular, the ability to predict the crack initiation from corrosion pit. To achieve this level of understanding, conducting fatigue tests in an environmental condition replicating the field environment is important. This paper presents the test protocol and results of environmental fatigue testing using bespoke laboratory apparatus to undertake in situ corrosion fatigue tests in a sour corrosive environment under uniaxial loading. The environment selected represent processes that are likely to occur at internal surfaces of oil and gas pipelines exposed to production fluids. The tests were carried out on smooth samples to obtain S‐N curve in this specific environment as well as on pre‐pitted samples. An electrochemical method is used to create corrosion pits on the samples. Also, a model is proposed to predict the crack initiation life from corrosion pit, using a local stress‐based technique, which has been validated by experimental test results. Post‐test fractography was carried out by scanning electron microscopy (SEM). The performance of our approach is demonstrated. The innovation is anticipated to encourage other workers to employ similar small‐scale tests requiring toxic and challenging test environments.

This study focuses on a better understanding of pitting and crevice corrosion on coating surface damaged carbon steels in automotive applications. Immersion and cyclic polarization tests were conducted on bare and coated metals in a 5% NaCl solution. The morphology of localized corrosion, including a filiform appearance, was characterized after exposure to the corrosive environment. To simulate the coating damage of automotive suspension coil springs and stabilizer bars under road conditions, a sharp-tipped scratching awl was used to create scratches and indents. Localized corrosion on the zinc phosphate (ZnP) pretreated and coating damaged surface was investigated in a 24-hour immersion test. The correlation between localized corrosion and metal substrate exposure is studied. Key factors affecting the localized corrosion of carbon steels with damaged coating are discussed.

Pitting, crevice and stress corrosion cracking are the most damaging corrosion forms of stainless steels in industrial applications. Generally, pitting and crevice susceptibility depends on a variety of factors related to the metal (chemical composition, differences in the metallurgical structure, inclusions), the environment (chloride content, pH, temperature, differential aeration) and the geometry of the system. Due to their unpredictable occurrence, localized corrosion events cannot be explained without using a proper statistical method. In this work a probabilistic approach based on Markov chains for the assessment of pitting and crevice corrosion initiation is proposed. A Markov chain is a stochastic process that undergoes transitions from one state to another through a finite number of possible states, until a so-called "absorbing state” from which the system has no tendency to evolve is attained. Formally, a Markov chain is characterized by a set of states and a transition probability matrix. The model calculates the probability to have pitting (and vice versa to maintain a stable passive condition) involving a large number of operating parameters related to both metal and environment. Experimental tests were carried out to validate the model which requires more accurate investigations.

The effect of nitrogen alloying on the pitting corrosion resistance of duplex and austenitic stainless steels has been examined. In order to avoid alteration of the phase ratio as a result of nitrogen alloying of the duplex steels, a simultaneous decrease has been made in the nickel content. Austenitic alloys of compositions corresponding to the austenite phase of the duplex steels have been investigated and compared to the behaviour of austenitic steels in which the nitrogen content or the nickel content alone has been varied. Nitrogen has a beneficial effect on pitting and crevice corrosion resistance in all cases but the duplex stainless steel exhibit a lower resistance to pitting and a higher resistance to crevice corrosion than predicted from the austenite nitrogen content.

The localized corrosion properties of lean duplex stainless steel (LDSS) and type 316L were compared by means of electrochemical testing and X-ray photoelectron spectroscopy (XPS) analysis. Electrochemical testing revealed that the pitting corrosion resistance of LDSS was superior to that of type 316L, but the crevice corrosion resistance of 316L was better than that of LDSS. In potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) analysis, as the pH decreased, the breakdown potential of type 316L did not change largely. However, the breakdown potential of LDSS sharply decreased at pH 4, and was lower than that of 316L at pH 2. Regardless of the pH, the passive film compositions of 316L and LDSS were the same, except for Mo compounds. Thus, the difference in the passive film stability of the two steels as the pH decreased was attributed to the difference in Mo content. Although pitting and crevice corrosion are similar in many respects, their initiation mechanisms differ. The resistance to these two types of corrosion can change depending on the concentration of Mo oxide in the passive film.

Adaptive Gaussian process regression as an alternative to FEM for prediction of stress intensity factor to assess fatigue degradation in offshore pipeline

Coupling an XFEM code for linear elastic fracture propagation to a viscous flow model. GeoComputation 2019

Detection of localized (crevice or pitting) corrosion is very difficult. Crevice attack occurs in shielded areas which cannot be observed visually, and equipment often fails without previous warning. Pitting is similar in that the visible pit area is often insignificant compared to the additional attack hidden below the surface. There is a variable incubation period before initiation of localized corrosion and apparatus which normally appears resistant can develop deleterious localized corrosion at any time.

Finite element analysis (FEA) has been undertaken to calculate the stress intensity factor (K) of cracks emerging from a corrosion pit in a martensitic stainless steel cylinder stressed remotely. The corrosion pit was in the form of a truncated spheroid with depth equal to the full mouth opening. This geometry reflects the configuration found in laboratory testing and in service in low chloride solution, albeit the pit is considered ideally smooth. The pit depth was fixed at 150 µm. A semi-circular crack was adopted with depths ranging from 25 µm to 250 µm. Computations were performed assuming purely elastic condition in most cases but the impact of plasticity around the pit was evaluated for selected crack depths. A key observation of this investigation is that cracks on either side of the pit that are small compared to the pit depth act independently with K values unaffected by the presence of the other crack. However, as the cracks on either side of the pit grow to a depth approaching the pit depth the K value is mutually increased. For cracks with depth close to that of the pit depth the stress intensity factor is increased due to the presence of the pit such that the expected increase of K with increasing crack size does not immediately materialise due to the decreasing stress gradient away from the pit. Elastic plastic analysis in discreet cases confirms these trends but highlights the need to avoid simple comparison of K values that may not be applicable because of plasticity around the pit.