Corrosion Damage Mechanisms Research Articles

Failure Analysis of α-MDEA Air-Cooled Heat Exchanger of CO2 Removal Unit Based on the Semiquantitative Risk-Based Inspection Method

Risk-based inspection program (RBI) is a decision making techniques for providing refineries and petrochemical plants with an effective and appropriate inspection planning. The aim of the present study is to analyze failure behavior and risk ranking of α-MDEA fin/fan cooler of CO2 removal unit based on API 581 in order to find out the reason(s) of tubes failure. According to previous experience in similar plant, risk calculation results and conducted inspection activity by using RFT eddy current technique, CO2 corrosion damage mechanism was identified as a main reason of tubes failure. In next step, process key variable, prevention/mitigation methods and inspection planning programs were determined so as to minimize the similar failure behavior in the rest of the lean amine air-cooled heat exchangers of CO2 removal unit.

Effect of Corrosion Damage on Fatigue Crack Initiation Mechanism and Growth Behavior of High Strength Aluminum Alloy

Corrosion damage are simulated through accelerated corrosion testing based on the complied environment spectrum, as result of high humidity atmosphere, salt fog, sulfur dioxide, acid rain and dry/wet alternation for aircraft structure during service.Prior corrosion and fatigue experiments are conducted for single edge notched specimen machined from 7B04-T74 advanced high strength aluminum alloy of aircraft critical structure. And the influence of different corrosion damage on fatigue crack initiation and propagation behavior is analyzed, as well as crack growth rate and fatigue life, to find the mechanism of corrosion damage on fatigue behavior. The results indicate that crack initiation site is mainly single pit in early corrosion stage, and the crack follows a relatively straight path. While the corrosion damage grows more severely, multiple small cracks initiate from several pits and there is obvious tunneling effect beneath pits, cracks follow zigzag path. The material microstructures compete with corrosion pits as crack origins.Corrosion damage seriously decreases fatigue life of aluminum alloy. Fatigue crack initiation life with corrosion damage equivalent to 12 years is only 2.2% of that without damage, which indicates crack initiation life is especially sensitive to corrosion damage.

Corrosion Damage Research of Shot Peening Aluminum Alloy

The pre-corrosion damage tests of LC4 aluminum alloy with the shot peening surface treatment and un-shot peening surface treatment are performed under two different time levels: 12h and 24h. Based on the observation of the corrosion morphology of aluminum alloy, the corrosion damage mechanism of shot peening aluminum alloy can be divided into three stages: intergranular corrosion occurs first, and then denudation generates, at last pitting corrosion occurs as denudation depth reaches a certain degree. For shot peening aluminum alloy, the denudation depth, mass loss and the total depth of corrosion damage are used to represent the corrosion characteristics. With the corrosion time increase, the three parameters all increase correspondingly.

Assessment of the electrochemical corrosion properties and environmentally induced cracking of an A106 Gr B steel pipe weld in a NaCl solution saturated with H2S gas

In the area of heavy construction, welding processes are vital in the production and maintenance of pipelines and power plants. The fusion welding process generates formidable welding residual stress and metallurgical change, which together increase the crack driving force and reduce the resistance against brittle fracturing and environmental fracturing. This is a serious problem with many alloys, and it also arises in A106 Gr B steel pipes. This type of piping, which is used in petrochemical and heavy chemical plants, either degrades due to the corrosive environment, e.g., those containing chlorides and sulfides, and/or become damaged during service due to various corrosion damage mechanisms. Thus, in this study, after numerical and experimental analyses of the welding residual stress of a multi-pass welded A106 Gr B steel pipe weld, the electrochemical corrosion properties and environmentally induced cracking of an A106 Gr B steel pipe weld were assessed in a 5.0 wt.% NaCl solution that was saturated with H2S gas at room temperature on the basis of NACE TM 0177-90. In terms of sulfide stress corrosion cracking (SSCC) and sulfide corrosion fatigue (SCF), the low SSCC limit of smooth specimens, σSSCCsmooth, and the SCF limit, ΔσSCF, were 46 % and 32 % (160 MPa) of the ultimate tensile strength (502 MPa) of an A106 Gr B steel pipe weld, respectively. Further, (Δσres)SCF was assessed under 75 MPa, which is 15 % of the tensile strength.

Assessment of the crack growth characteristics at the low corrosion fatigue limit of an A106 Gr b steel pipe weld

In the area of heavy construction, welding processes are vital in the production and maintenance of pipelines and power plants. The fusion welding process generates formidable welding residual stresses and metallurgical change, which increase the crack driving force and reduce the resistance of brittle fracture as well as environmental fracture. This is a serious problem with many alloys as well as A106 Gr B steel pipe. This pipe, used in petrochemical and heavy chemical plants, either degrades due to corrosive environments, e.g., chlorides and sulfides, and/or becomes damaged during service due to the various corrosion damage mechanisms. Thus, in this study, the sulfide corrosion fatigue strength of multi-pass welded A106 Gr B steel pipe was evaluated in a 5.0 wt.% NaCl solution that was saturated with H2S gas at room temperature on the basis of NACE TM 0177-90. The crack growth characteristics of the welded pipe were then assessed at the low limit of sulfide corrosion fatigue strength, which was previously obtained from the sulfide corrosion fatigue (SCF) tests. From the results, in terms of the SCF, all the specimens failed at the heat-affected zone, where a high welding residual stress distributes. It was found that the fatigue crack grew at the low corrosion fatigue limit (σSCFun-notched), which was 32 % (160MPa) of the ultimate tensile strength (502MPa) of the welded specimens.

Effect of the powder deposition rate on the mechanical properties and the structure of the APS-NiCr/Al coating

Effect of the powder deposition rate on the mechanical properties and the structure of the APS-NiCr/Al coating

Assessment of the sulfide corrosion fatigue strength for a multi-pass welded A106 Gr B steel pipe below the low SSCC limit

In the area of heavy construction, welding processes are vital in the production and maintenance of pipelines and power plants. Welding processes happen to produce residual stresses and change the metal structure as a result of the large nonlinear thermal loading that is created by a moving heat source. The fusion welding process generates formidable welding residual stresses and metallurgical change, which increase the crack driving force and reduce the resistance to the brittle fracture as well as the environmental fracture. This is a serious problem with many alloys as well as the A106 Gr B steel pipe. This pipe that is used in petrochemical and heavy chemical plants either degrades due to corrosive environments, e.g., chlorides and sulfides, and/or become damaged during service due to the various corrosion damage mechanisms. Thus, in this study, after numerically and experimentally analyzing the welding residual stress of a multi-pass welded A106 Gr B steel pipe, the sulfide stress corrosion cracking (SSCC) characteristics were assessed in a 3.5 wt.% NaCl solution that was saturated with H2S gas at room temperature on the basis of NACE TM 0177-90. The specimens used are of two kinds: un-notched and notched. Then, the sulfide corrosion fatigue (SCF) strength for the un-notched specimen was assessed below the low SSCC limit that was previously obtained from the SSCC tests for the notched specimen. From the results, in terms of the SSCC and SCF, all the specimens failed at the heat-affected zone, where a high welding residual stress is distributed. It was found that the low SSCC limit of un-notched specimens (σSSCCun-notched) was 46% (230 MPa) of the ultimate tensile strength (σU=502 MPa) of a multi-pass welded A106 Gr B steel pipe, and the notched specimens (σSSCCnotched) had 40% (200 MPa) of the ultimate tensile strength. Thus, it was determined that σSSCCun-notched was 13% lower than σSSCCnotched. Further, the sulfide corrosion fatigue limit (σSCFun-notched) was 32% (160 MPa) of the ultimate tensile strength of welded specimens. This σSCF un-notched was 20% lower than σSSCCnotched.

Optimal maintenance decisions under imperfect inspection

The process industry is increasingly making use of Risk Based Inspection (RBI) techniques to develop cost and/or safety optimal inspection plans. This paper proposes an adaptive Bayesian decision model to determine these optimal inspection plans under uncertain deterioration. It uses the gamma stochastic process to model the corrosion damage mechanism and Bayes’ theorem to update prior knowledge over the corrosion rate with imperfect wall thickness measurements. This is very important in the process industry as current non-destructive inspection techniques are not capable of measuring the exact material thickness, nor can these inspections cover the total surface area of the component. The decision model finds a periodic inspection and replacement policy, which minimizes the expected average costs per year. The failure condition is assumed to be random and depends on uncertain operation conditions and material properties. The combined deterioration and decision model is illustrated by an example using actual plant data of a pressurized steel vessel.

Effect of the Quantity, Composition, and Thickness of Deposits on the Damage to the Pipes of Steam Generators in Nuclear Power Plants with VVÉR Reactors

The mechanism of corrosion damage to the heat-transfer pipes of steam generators and the effect of the composition, thickness, and quantity of deposits on the risk of damage to the piping of steam generators are examined.

Corrosion and erosion damage mechanisms during erosion–corrosion of WC–Co–Cr cermet coatings

Experiments were performed in order to evaluate the behaviour of WC–Co–Cr coatings applied by the Super D-Gun™ (Trademark of Praxair S.T. Technology) thermal spray process in erosion–corrosion environments. The experiments were performed in 3.5% NaCl with different silica sand loadings (200 and 500 mg/l). The results were compared with UNS S31603 and UNS S32760 stainless steels. Measurements of the total material loss showed that the coating applied by Super D-Gun presented higher resistance to erosion–corrosion compared to both stainless steel materials as expected. The connection between the erosion–corrosion behaviour of the Super D-Gun coating and the microstructure is assessed. Scanning electron microscopy (SEM) images enabled the different mechanisms of tribo-corrosion to be understood. With cermet materials there has been much discussion relating to the role of corrosion in removing the hard phase particles which can breach tribological performance. The corrosion rate ( i corr) of the coating was determined under erosion conditions and an evaluation of the influence of corrosion and synergistic processes on the erosion process was made using applied cathodic protection.

Influence of deposition temperature and time during PVD coating of CrN on corrosive wear in liquid aluminium

Erosive and corrosive wear are two major life-limiting factors in die casting dies. To resist the corrosive and erosive attack from molten metal flow the tool surface needs to be hard and chemically inert. It has been indicated that local coating defects rather than intrinsic deficiencies limit the potential gain of coatings on hot work tool steel exposed to liquid aluminium. This motivates a search for suitable protective coatings on the dies. A number of ceramic coatings are of interest. In this work, physically vapour deposited (PVD) CrN coatings were applied on hot work tool steel specimens and treated in an aluminium melt. Substrate temperature and deposition time were varied to give coatings with individual properties. Type and density of defects as well as thickness of the coatings were characterised for all coatings. The influence of substrate temperature and deposition time on coating characteristics and corrosion resistance was studied as well as the mechanism of corrosion damage. It is clearly demonstrated that liquid aluminium corrosion of CrN-coated tool steel is initiated at defects which penetrate through the coating, and localised corrosion pits are formed. Subsequently, the pits coalesce and the corrosive attack aggravates. Consequently, the corrosion resistance is improved by reducing the density of defects through the coating, which, for PVD CrN, was achieved by increasing the coating thickness.

Relating Almen intensity to residual stresses induced by shot peening: a numerical approach

Shot peening is a surface impact treatment widely used to improve the performance of metal parts and welded details subjected to fatigue loading, contact fatigue, stress corrosion and other damage mechanisms. The better performance of the peened parts is mainly due to the residual stresses resulting from the plastic deformation of the surface layers of the material caused by the impact of the shot. Shot peening intensity is usually quantified by means of the Almen-scale, which measures the residual arc height of a strip made of a specific material, and of a pre-defined size. The scale does not, in other words, apply solely to the residual stress field of a component of unspecified material and size. In this paper, a finite element to predict the residual stresses induced by shot peening in a metal part and to relate these stresses to Almen intensity is proposed; the aim is to provide the designer with a useful tool with which to choose the optimal treatment parameters with respect to the mechanical behaviour of the peened parts. Experimental measurements of residual stresses and a comparison with existing experimental data validate this approach.

Mechanism of localized corrosion damage of the protective coating on a heat-resistant alloy

Mechanism of localized corrosion damage of the protective coating on a heat-resistant alloy