Investigation of Corrosion in Bolted Flanged Joints Using a Novel Experimental Setup
Authors: Hakimian Soroosh , Hof Lucas , Bouzid Hakim A. 
Published: July 2022

The second most common cause of hydrocarbon leakage is corrosion in offshore platforms. In seawater and hydrocarbon services, bolted flange joints can be susceptible to corrosion on the flange face. Flanges connect pipes, valves, pumps, and other equipment to form a piping system. In flanged joints, two flanges are bolted together with a gasket between them to provide a seal. The current research considers corrosion in bolted flanged gasketed joints and proposes a new approach to quantify corrosion at the gasket and flange interface. According to the literature, both crevice corrosion and galvanic corrosion widely occur in bolted flanged gasketed connections, which create paths to leakage of the pressurized fluid. Leakage failure in bolted flanged gasketed joints can cause hazards to the environment and human safety. Corrosion in bolted gasketed joints was investigated in other papers and reports. Still, these studies did not consider the influence of the operating conditions. Fluid flow, pressure, pH, conductivity, temperature, and gasket contact pressure are some parameters to consider.

To achieve this goal, a novel experimental setup - mimicking an industry-standard NPS 2 bolted flange connection - is introduced to examine the corrosion behavior of the contact surface between the flange and gasket. The setup enables monitoring and recording of the corrosion parameters under the influence of service conditions (gasket contact pressure, pH, salinity, temperature, fluid flow, and fluid pressure) during the corrosion tests. In a second step, the influence of the service conditions on corrosion will be studied. Two levels of temperature (25 o C and 85 o C), gasket contact pressure (4 MPa and 35 MPa), and flow rate (1.4 cm. s-1 and 4.5 cm. s-1) are considered in the study. The flange material selected in this study is stainless steel 304/304L since it has a wide application in the industry. Flexible graphite materials, commonly used due to their chemical resistance, high-temperature capability, low cost, and good sealing properties, are used in the corrosion tests. Two direct current electrochemical test techniques, polarization resistance (Rp) and potentiodynamic anodic polarization, are carried out, according to ASTM G5, to measure the corrosion rate in addition to other quantification methods based on visual observations and mass loss. These tests are conducted using the novel designed setup that consists of a working electrode (flange material), a reference electrode (Ag/AgCl), and an auxiliary electrode (a stainless-steel rod). The synthetic seawater solution recommended in ASTM D1141 is used for the corrosion tests. In a final step, the corroded surfaces of the specimens are examined via some of the following available techniques; confocal laser microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray microscopy (EDS/EDX), and X-ray powder diffraction (XRD) techniques. Confocal microscopy is used to visualize the morphology of the damaged zones on the surface, and localize and quantify the crevices volume caused by corrosion, respectively. SEM and XRD analysis can reveal the scale morphology and phases present on the corroded plate surfaces. Comparing the results of the electrochemical tests and the microscopic studies will identify the influence of operating factors on the corrosion at the gasket-flange interface.

Preliminary tests were performed to check the applicability of the novel setup for studying corrosion behaviour between gaskets and flanges and solve initial setup problems to get reproducible results. In the current research, the corrosion behaviour is studied under dynamic conditions. The majority of other works consider only the static condition, so it is important to run some preliminary tests to shakedown the rig and fixture and resolve unanticipated issues. First attempts failed due to noisy polarization curves that could not be used for corrosion analysis. After several experiments on small samples, it was decided to use filter paper immersed in a 3.5% NaCl solution as a salt bridge to reduce the distance between the reference and working electrodes in the setup. This resulted in smooth polarization curves useful for further corrosion analysis (Fig. 1).

The novel designed bolted flange joint setup allows corrosion monitoring and characterization under dynamic operating conditions for the first time.

Figure 1