Quality analysis of galvanized pipes applied to natural gas installations in civil construction

Some natural gas distributors for residential and commercial buildings in Brazil use galvanized steel tubesand fittings manufactured by the hot-dip zinc process that are supported by the specifications contained in theBrazilian Standards. Zinc coating is one of the most widely used methods to provide protection againstcorrosion. It is therefore essential that national and international tube manufacturers and installation andmaintenance companies in civil construction also meet the defined national and international standards,especially when the tubes and fittings are used for natural gas in residential buildings. This two-waycommunication could eliminate most issues that delay or prevent superior galvanizing quality. Low standardslead to the deterioration of the zinc coating and the metal base of the pipes, which may lead to leaks, such asfluids, flammable gases, and other toxic products, and potentially jeopardise residential and commercialbuildings. Here the importance of ensuring the quality of zinc-coated pipes is demonstrated through a seriesof analyses on natural gas pipe corrosion used in civil construction in Brazil. Based on Brazilian standards,the zinc mass determination per square meter, uniformity tests (Preece test) and metallographic evaluation ofthe deposited zinc layer were carried out on samples of galvanized tubes for natural gas. An experiment wasalso developed to reproduce the same conditions of subfloors (mortar beds) where there is penetration ofwater and humidity reaching the galvanized pipes. Metallographic analysis showed that samples of galvanizedtubes by the hot-dip zinc galvanizing can have discontinuities and defects in zinc layer. Such defectsassociated with penetration and/or permeation of the water and humidity in the subfloor favor the corrosiveprocess and consequently can cause gas leaks and probable accidents in residential buildings.Keywords: Corrosion, galvanized steel, zinc coating, quality, natural gas pipes, civil construction

Análise comparativa das propriedades mecânicas e resistência à corrosão do aço AISI 304l soldado por GTAW convencional e pulsado: Comparative analysis of mechanical properties and corrosion resistance of AISI 304l steel welded by conventional and pulsed GTAW processes

O aço AISI 304L é especificado como metal de base para juntas soldadas em equipamentos diversos, devido à boa resistência à sensitização. Em algumas aplicações a temperatura de serviço ultrapassa 450°C. O processo GTAW pulsado é uma alternativa ao convencional, por amenizar os efeitos prejudiciais dos sucessivos ciclos térmicos em soldagens multipasses. Entretanto, requer maiores investimentos, gerando questionamentos sobre os ganhos metalúrgicos dessa alternativa. Nesse contexto, esse trabalho comparou a resistência à corrosão por pites e a tenacidade do Aço AISI 304L soldado pelos dois processos. Para isso, foi realizado o envelhecimento térmico na temperatura de 450°C pelo período de até 24 horas e, posteriormente, foram realizadas caracterizações microestruturais por microscopia ótica (MO) e eletrônica de varredura (MEV-EDS), e permeabilidade magnética. As propriedades mecânicas foram avaliadas por ensaios de microdureza e de tenacidade ao impacto nas juntas metálicas, e comparadas entre si nas condições de envelhecimentos indicadas. A avaliação da susceptibilidade do material à corrosão por pites foi realizada pela técnica de polarização potenciodinâmica cíclica em solução aquosa contendo 3,5% de NaCl. Os resultados indicaram valores de tenacidade maiores nas amostras de GTAW pulsado para todas as condições propostas. Além disso, o maior tempo de envelhecimento provocou queda da tenacidade, possivelmente por conta da formação de fases intermetálicas. Entretanto, na condição de como soldado, o GTAW convencional apresentou maior resistência à corrosão por pites. Os resultados, portanto, sugerem que a escolha entre os processos considere, primordialmente, fatores econômicos

Influence of Microstructure on the Corrosion Resistance of AISI 317L (UNS S31703)

<div><p>The AISI 317L stainless steel is an austenitic grade with at least 3%Mo. Recently, this steel has replaced AISI 316L in many applications in chemical and petrochemical industries, due to its higher pitting corrosion resistance. The microstructure of the hot rolled and annealed material studied in this work consists of austenitic grains and 4.0% of delta ferrite (δ) with elongated islands morphology. This microstructure was modified by three processes: cold rolling with 87% of reduction, aging at 450ºC for 400 h, and welding by gas tungsten arc welding (GTAW) process. The corrosion resistance was evaluated by anodic polarization tests (ASTM G-61) and critical pitting temperature tests (ASTM G-150). Cold rolling produced a microstructure consisting of elongated grains of austenite and martensite α', high dislocation density and the elongated islands of delta ferrite present in the annealed material. Welding produced a dendritic microstructure with 7.0% of delta ferrite and some σ precipitated in the ferrite. Finally, the aging at 450ºC for 400 h provoked the decomposition of ferrite. The results show that these microstructural changes affected the pitting corrosion resistance, as detected by ASTM G-61 and ASTM G-150 tests.</p></div

Effects of Aging at 450°C on the Pitting Corrosion Resistance and Toughness of AISI 317L Steel Welded by GTAW and FSW

<div><p>Austenitic stainless steels are high corrosion resistant alloys widely used in many industrial fields. Among this family of steels, AISI 317L stands out due to its higher localized corrosion resistance when compared to the traditional grades AISI 304L and AISI 316L. In some applications in oil refineries, the AISI 317L is being specified for services at moderately high temperatures. At the same time as it is sought to use new stainless steels, is also desirable to apply and develop emerging welding processes, replacing conventional ones, in order to achieve better behavior in service. In this respect, this work studied the effects of thermal aging on the toughness and resistance to pitting corrosion of AISI 317L steel weld metals produced by the Gas Tungsten Arc Welding (GTAW) and Friction Stir Welding (FSW) joining processes. After prolonged exposures at 450°C, for 200h, 300h and 400h, the microstructural characterization by scanning electron microscopy (SEM), toughness evaluation and anodic polarization tests in 3.5% NaCl solution were performed. The results showed that the increase of the exposure time in both weld metals caused a toughness decrease. The pitting potentials measured in the polarization tests also decreased with the aging at 450ºC.</p></div