Process equipment which employs a corrosion resistant alloy (CRA) layer cladded to steel is common in refineries, petrochemical plants and other plants processing highly corrosive media. There are two regularly employed methods for welding attachments and internals to clad process vessels. One is to remove the CRA cladding for welding the attachment to the steel base metal assuming dissimilar welds and restoring CRA by weld overlay. The other eliminates the step of removing the cladding, simplifying the attachment process by direct welding of the internals onto the clad layer. With the lack of data to prove the integrity of direct welding attachment onto the clad layer, designers frequently demand the cladding be removed or allow only a conservatively low stress limit for what can be attached directly to the clad surface. It is well understood that eliminating the step of removing clad increases the simplicity, improves the lead-time, and reduces the cost of making these attachments for trays or other internals, but there are concerns about clad disbonding risks. With the aim to provide data around the integrity of direct welding attachments for better risk assessments, a technical study was undertaken. In this study, it will be shown that the bond between clad material and the base steel is robust enough to withstand the heaviest attachments and harshest conditions. The theory behind the technical study will be presented along with the results of this study

With increasing oil & gas demand and depletion of sweet reserves, oil & gas companies in the regional
economies are focusing towards the exploitation of sour resources. This necessitates the use of pipelines
and down-hole tubing made from special steels with significant resistance to hydrogen-induced cracking
(HIC). These steels are produced through specific technologies for enhanced chemical composition control
and microstructural engineering to incorporate the required strength, weld ability and improved HIC
resistance. It is well established that the HIC initiates at sites with microstructural heterogeneities whether
due to presence of gross nonmetallic inclusions or the micro-structural constituents. The presence of central
segregation further aggravates the conditions particularly when the final pipe sizes require the longitudinal
slitting of the coils. Presence of non-metallic inclusions in the steel makes it vulnerable to hydrogen-induced
cracking under wet H2S environment. The mechanism of HIC begins with the generation of hydrogen atoms
by corrosion reaction of H2S and Fe in the presence of free water. The hydrogen atoms then diffuse into
steel and accumulate around the inclusions. The higher number of inclusions equates to the more sites
available for hydrogen adsorption. Recombination of hydrogen atoms to H2 molecules builds up a heavy
gas pressure in the interface between matrix and inclusions. Cracking initiates because of the tensile stress
field caused by hydrogen gas pressure and crack propagates in the surrounding steel matrix. The
longitudinal slitting exposes the internal microstructural abnormalities to the skelp edges which are then
incorporated in the thermally stressed weld zone. While the post-weld heat treatment (PWHT) mostly
homogenizes the weld zone microstructure, the presence of excessive central line features cannot be
completely removed thereby making this zone more prone to HIC attack

A test programme was conducted to study corrosion behaviour of - 1. carbon and low alloy steels, 2. austenitic, martensitic and duplex stainless steels and 3. nickel-based Alloy 718 - in low oxygen content (20 & 200 ppb) seawater.