Carlos Alberto Della Rovere

Evaluation of Anodic Behavior of Commercially Pure Titanium in Tungsten Inert Gas and Laser Welds

PURPOSE This study evaluated the resistance to corrosion in welds made with Tungsten Inert Gas (TIG) in specimens made of commercially pure titanium (cp Ti) in comparison with laser welds. MATERIALS AND METHODS A total of 15 circular specimens (10-mm diameter, 2-mm thick) were fabricated and divided into two groups: control group-cp Ti specimens (n = 5); experimental group-cp Ti specimens welded with TIG (n = 5) and with laser (n = 5). They were polished mechanically, washed with isopropyl alcohol, and dried with a drier. In the anodic potentiodynamic polarization assay, measurements were taken using a potentiostat/galvanostat in addition to CorrWare software for data acquisition and CorrView for data visualization and treatment. Three curves were made for each working electrode. Corrosion potential values were statistically analyzed by the Students t-test. RESULTS Statistical analysis showed that corrosion potentials and passive current densities of specimens welded with TIG are similar to those of the control group, and had lower values than laser welding. TIG welding provided higher resistance to corrosion than laser welding. CONCLUSION Control specimens welded with TIG were more resistant to local corrosion initiation and propagation than those with laser welding, indicating a higher rate of formation and growth of passive film thickness on the surfaces of these alloys than on specimens welded with laser, making it more difficult for corrosion to occur.

Effect of Cooling Rate on the Corrosion Behavior of As-Cast SAF 2205 Duplex Stainless Steel After Solution Annealing Treatment

The formation of intermetallic phases alters the properties of duplex stainless steel and is therefore a crucial factor in its performance. For example, the formation of sigma phase in duplex steel increases its brittleness and decreases its corrosion resistance because this phase consumes chromium and molybdenum in solid solution, thus leading to the depletion of these elements in the matrix. This study investigated the corrosion resistance of as-cast SAF 2205 duplex stainless steel after solution annealing treatment at 1100 °C for 240 minutes, under varying cooling rates. The objective was to evaluate the correlation between cooling condition, microstructural changes and corrosion resistance based on cyclic potentiodynamic polarization and double loop electrochemical potentiokinetic reactivation (DL-EPR) measurements. The results revealed a significant reduction in the corrosion resistance of a slowly cooled sample, which presented an increase in the degree of Cr (Qr / Qa) depletion resulting from the formation of sigma phase.

Efeito do revenido na resistência à corrosão dos aços inoxidáveis supermartensíticos

Supermartensitic stainless steels have been used in the oil and gas industry for onshore and offshore tubing applications, due to their good mechanical properties, weldability and excellent corrosion resistance. They are hardened by quenching heat treatment, and to improve their toughness, are submitted to tempering. During the tempering, some phase precipitation occurs, which depending on the time and temperature parameters, produces some undesired changes in the steel properties. The aim of this research was to study the microstructure and the corrosion resistance of supermartensitic stainless steel in quenched and different tempered conditions (550°C, 600°C and 650°C). At the microstructure of the 650°C tempered sample was observed the formation of austenite and precipitates of chromium, like Cr3C2 and Cr7C3.The polarization curves indicated that the heat treatment influences the anodic behavior, changing the passive region and the passive film characteristics. The tempering changes the corrosion resistance, decreasing the degree of sensitization when increasing the temperature from 550°C to 650°C, this occurs due to the recovery of the chromium impoverished zones.

Corrosion behaviour of a dissimilar joint TIG weld between austenitic AISI 316L and ferritic AISI 444 stainless steels

Abstract The AISI 444 stainless steel (SS) has become an option to substitute the AISI 316L SS because of its low cost and satisfactory corrosion resistance. However, the use of AISI 444 alloy tubes in heat exchangers causes the welding of a dissimilar joint. The aim of this study was evaluate the corrosion resistance of the tube-to-tubesheet welded by a TIG process composed of AISI 316L and AISI 444. Preparation of samples was executed through replication of tube-to-tubesheet joints. In order to test the corrosion resistance of the welded joint, the following tests were applied: sensitisation, mass loss from room temperature up to 90 °C and electrochemical corrosion tests in 0.5 mol/L HCl and 0.5 mol/L H2SO4 electrolytes. The results have shown that the dissimilar joint suffers galvanic corrosion with increased degradation of the heat-affected zone of the AISI 444 tube. Nevertheless, the mechanisms of localised corrosion (pit and intergranular) were more active in the AISI 316L alloy. It is concluded that the dissimilar joint showed better corrosion resistance than the welded joint composed solely of AISI 316L at temperatures up to 70 °C, as the conditions observed in this work.

Microstructure and corrosion resistance of radial friction welded supermartensitic stainless steels

Abstract In this work, supermartensitic stainless steel pipes were radial friction (RF) welded and their corrosion behaviours were studied based on potentiodynamic polarisation and double-loop electrochemical potentiokinetic reactivation tests. Measurements were performed on samples taken from the base metal (BM), weld interface and consumable ring (CR) of the RF weldment. The corrosion properties are discussed in terms of their resulting metallurgical microstructure. The precipitation of Cr carbides that takes place during the tempering treatment induces a substantial Cr depletion value (Ir/Ia = 54.22%). On the other hand, CR and weld interface regions, which had their microstructure transformed and their Cr carbide precipitates redissolved by the RF welding thermomechanical cycle, present a low level of Cr depletion (Ir/Ia < 1%). The AC microstructure, which is composed of a mixture of virgin martensite and stable retained austenite, presents an increase in pitting corrosion resistance compared to the tempered structure of the BM region. It was also observed that the δ-ferrite decreases the pitting resistance of the weld interface region.

Effect of Thermal Aging at Low Temperature on the Mechanical Properties and Corrosion Resistance of LDX 2404 Duplex Stainless Steel

It is well known that when duplex stainless steels (DSS) are subjected to temperatures ranging from 300 to 1000 °C they may undergo precipitation of several phases, which can seriously impair their mechanical properties and corrosion resistance. The present work studied the effect of thermal aging (up to 2000 h) at 475 °C on the corrosion and mechanical properties of the newly developed 2404 DSS. The evaluation was based on potentiodynamic polarization in 3.5% NaCl solution and on Charpy tests. The pitting corrosion resistance was found to decrease significantly with aging time at 475 °C. In addition, Charpy tests revealed that after 100 h of thermal aging the material becomes brittle due to the spinodal decomposition of ferrite.

Corrosion Behavior of Fe-Mn-Si-Cr-Ni-Co Shape Memory Stainless Steel in Highly Oxidizing Medium

A study was conducted on the corrosion behavior and characteristics of the passive oxide film of Fe-Mn-Si-Cr-Ni-(Co) shape memory stainless steels (SMSS) in a concentrated nitric acid (HNO3) solution, based on potentiodynamic polarization, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses. The results indicated that Fe-Mn-Si-Cr-Ni-(Co) SMSSs exhibit a passive behavior similar to that of 304L austenitic stainless steel (304L SS). However, unlike 304L SS, their high silicon (Si) content renders them insensitive to intergranular attack in highly oxidizing environments. The XPS analysis also indicated that Si appears to be the main element responsible for the high protectiveness afforded by the passive film formed on Fe–Mn–Si–Cr–Ni–Co SMSS.

Effects of Peracetic Acid on the Corrosion Resistance of Commercially Pure Titanium (grade 4)

The aim of this study was to evaluate the corrosion resistance of pure titanium grade 4 (cp-Ti-4), subjected to disinfection with 0.2% and 2% peracetic acid during different immersion periods using anodic potentiodynamic polarization test in acid and neutral artificial saliva. Cylindrical samples of cp-Ti-4 (5 mm x 5 mm) were used to fabricate 24 working electrodes, which were mechanically polished and divided into eight groups (n=3) for disinfection in 2% and 0.2% peracetic acid for 30 and 120 min. After disinfection, anodic polarization was performed in artificial saliva with pH 4.8 and 6.8 to assess the electrochemical behavior of the electrodes. A conventional electrochemical cell, constituting a reference electrode, a platinum counter electrode, and the working electrode (cp-Ti specimens) were used with a scanning rate of 1 mV/s. Three curves were obtained for each working electrode, and corrosion was characterized by using scanning electron microscopy (SEM) and energy dispersive x-ray spectrometry (EDS). Data of corrosion potential (Ecorr) and passive current (Ipass) obtained by the polarization curves were analyzed statistically by Students t-test (a=0.05). The statistical analysis showed no significant differences (p>0.05) between artificial saliva types at different concentrations and periods of disinfection, as well as between control and experimental groups. No surface changes were observed in all groups evaluated. In conclusion, disinfection with 0.2% and 2% peracetic acid concentrations did not cause corrosion in samples manufactured with cp-Ti-4.

Resistência à corrosão de junta dissimilar soldada pelo processo TIG composta pelos aços inoxidáveis AISI 316L e AISI 444

O aco inoxidavel AISI 444 tornou-se uma opcao para substituir a liga AISI 316L devido ao seu menor custo e satisfatoria resistencia a corrosao. Entretanto, o uso da liga AISI 444 no feixe tubular de trocadores de calor acarreta na soldagem de uma junta dissimilar. O presente estudo teve por objetivo avaliar a resistencia a corrosao da junta tubo-espelho soldada pelo processo TIG composta pelas ligas AISI 316L e AISI 444. A manufatura das amostras consistiu em replicar o projeto da junta tubo-espelho de trocadores de calor. Realizou-se em juntas soldadas ensaios de sensitizacao, perda de massa por imersao desde a temperatura ambiente ate 90 oC, e ensaios eletroquimicos de polarizacao potenciodinâmica nos eletrolitos 0,5 mol/L de HCl e 0,5 mol/L de H2SO4. Os resultados mostraram que a junta dissimilar sofreu corrosao galvânica com maior degradacao na zona afetada pelo calor (ZAC) do tubo AISI 444. Porem, os mecanismos de corrosao localizada (pite e intergranular) demonstraram ser mais ativos para a liga AISI 316L. Conclui-se que a junta dissimilar apresentou melhor resistencia a corrosao do que a junta soldada composta unicamente pela liga AISI 316L em temperaturas de ate 70 oC, conforme as condicoes observadas neste trabalho.

Microestrutura e resistência à corrosão de aços inoxidáveis supermartensíticos soldados por fricção radial

Neste trabalho, tubos de aco inoxidavel supermartensitico (AISM) foram soldados por friccao radial (SFR) e suas propriedades de corrosao foram estudadas atraves de ensaios de polarizacao anodica em solucao de 3,5 % de NaCl e da tecnica eletroquimica de reativacao potenciocinetica de ciclo duplo (DL-EPR). As medidas foram realizadas em amostras extraidas de diferentes regioes da solda SFR, i.e., metal de base (MB), interface de soldagem e anel consumivel (AC). As propriedades de corrosao foram discutidas em termos da microestrutura metalurgica resultante em cada regiao. A precipitacao de carbonetos de Cr que ocorre durante o tratamento de revenimento causa um empobrecimento em Cr substancial na estrutura revenida do MB (Ir/Ia = 54,22%). Por outro lado, as regioes do AC e da interface de soldagem, que tiveram suas microestruturas transformadas e seus precipitados de carboneto de Cr redissolvidos pelo ciclo termomecânico de soldagem SFR, apresentam um baixo grau de empobrecimento em Cr (Ir/Ia < 1%). A microestrutura do AC, composta de uma mistura de martensita temperada e austenita retida, apresenta maior resistencia a corrosao localizada do que a estrutura revenida do MB. Tambem foi observado que a ferrita-δ diminui a resistencia a corrosao por pites na regiao da interface de soldagem.