The Effect Of Cathodic Protection On The Tensile Ductility Of Case Node Steels
Authors: Kermani M.B. 
Published: October 2008

The susceptibility of cathodically protected prototype cast node steel specimens to hydrogen embrittlement has been investigated in aerated 3.5% NaCl solution using Slow monotonic straining and static loading. In addition, fracture surfaces have been examined under a scanning electron microscope.

/t has been shown that embrittlement occurs at potentials more negative than a transition potential, found to be between -850 to -900 mVSCE. At potentials more positive than the transition potential, the ductility remains unchanged and is similar to the values obtained in air. further lowering of potential to more active potentials causes a rapid decrease in ductility. The presence of casting defects within the microstructure, not only has a detrimental influence on the ductility, it also slightly increases the transition potential to more positive values. The observation of brittle fracture and acceleration in fatigue crack growth rate are consistent with a hydrogen embrittlement mechanism resulting from the presence of catholically evolved hydrogen.

The results presented, in addition to assisting the recognition of cracking mechanism of C-Mn steels, provide significant practical implications through distinguishing a safe and effective level of cathodic protection to eliminate detrimental side effects of overprotection.

INTRODUCTION

Oil and gas offshore installations are constructed from low strength structural steels such as BS 4360 grade 50D. These structures are usually fabricated from welded tubular members which are intersected through welded node joints. Such joints are conventionally made by welding. Welded nodes not only promote a load concentration at the weld toe, their fabrication and inspection create practical problems. The difficulties with the large complex welded nodes may be overcome be using cast joints with smooth contoured intersections 1,2. The principal advantages of cast nodes are that, being more effectively designed, they improve fatigue life, reduce stress concentration and reduce net section stresses by increasing section thickness1,2.

An appreciation of the combined effects of slow monotonic straining and cathodic potential on the embrittlement of low strength C-Mn steels exposed to saline environments has only been made recently3â??4. In these systems, The loss in ductility has been primarily attributed to the formation of transgranular cracks on freshly created metals arising from continuous straining at the presence of catholically evolved hydrogen3â??4. These materials were previously reported5to be immune to environmentally-induced cracking under constant load conditions, although a large number of publications now exist which refers to the influence of strain-rate and potential on the ductility of pipeline steels in carbonate- bicarbonate solutions5–7.

In the work presented in this paper three principal objectives have been met. First the effect of cathodic potentials on the tensile ductility of a prototype cast steel node exposed to continuously aerated 3.57. sodium chloride solution has been examined and in particular a range of potential where safe application of cathodic protection is possible has been determined. Secondly, fractographic analysis has been carried out, on the basis of which a cracking mechanism has been deduced. Finally, the presence of casting defects and microporosity and their influence on the ductility behaviour has been studied.