K A Alawadhi - Preferntial corrosion of carbon steel weld studiesin a flow channel - страница 1
Alawadhi K.A. &Al-Ali A.Y.
PREFERNTIAL CORROSION OF CARBON STEEL WELD STUDIES
IN A FLOW CHANNEL
The aim of this research is to investigate the cause of the severe localised corrosion that sometimes occurs at welds in carbon steel pipelines carrying hydrocarbons and inhibited brines saturated with carbon dioxide. A flow Channel Apparatus at the lab was used to simulates the actual condition that occurs in the pipeline. Different Samples from the parent metal, weld metal and heat affected zone pipeline steel could be galvanically coupled and tested under static and flowing conditions with and without inhibitor. The total corrosion rate of each weld region was recorded from the sum of the self-corrosion and galvanic contributions. A secondary aim of the research is to investigate under which conditions current reversal takes place. Five references been used.
Keywords: weld corrosion, carbon steel, inhibitor, carbon dioxide, flow channel.
Introduction. In oil and gas industry, localised corrosion is the most serious and frequent cause of pipeline failure usually occur not as a result of uniform corrosion but from localised attack, generally as pitting or galvanic corrosion of welds. These forms of attack can cause pipelines down-hole tool degradation that can lead to spillage of products resulting in negative environmental impact Therefore, they place high demands upon the corrosion inhibitors, which need to remain effective under high flow rates and turbulent operating conditions.
There is an extensive use of carbon steels as materials of construction for pipes in the oil and gas industries. These steels are made of cheap materials of construction used for many applications and therefore involve lesser capital investment, but they usually exhibit poor corrosion resistance properties as a result internal corrosion.
The consumables used in pipeline welding are generally selected so as to confer slightly cathodic properties on the weld metal and avoid the most severe problems of localised weld corrosion. This has sometimes been achieved  by the addition of 1%Ni. However, in some cases, the use of inhibitors to control internal pipeline corrosion has the effect of reversing the galvanic currents between the weld regions such that the weld metal becomes anodic and accelerated rates of galvanic corrosion takes place. Examples of this severe preferential weld corrosion  see Figure 1. These problems are heightened by the small surface areas of the weld metal and heat-affected zones compared to the large area of the parent material.
Figure 1 an example of severe preferential weld corrosion in carbon steel pipeline carrying inhibited oil and gas 
Experiments. All tests were carried out on samples machined from welded X65 steel pipe with compositions of 0.08%C, 1.6%Mn & 0.3%Si.The steel had been thermo mechanically controlled rolled to give a 32mm thickness with a fine-grained ferritic microstructure and a hardness of 200-210 Hv. The double-vee weld had been produced by the submerged arc process, with a relatively high heat input (5-10 J/mm) and a hardness of 223230 Hv. A sample was cut from the weld, polished and etched to identify the positions of the parent material, heat affected zones and weld metal, as shown in Figure 2.
Figure 2 Sections of the weld (parent material PM, heat affected zone HAZ and weld metal WM)
Flow Channel Apparatus. Electrochemical corrosion rate measurements were carried out using a flow channel produced from the different regions of the weld. An advantage of using flow channel is that the hydrodynamic conditions are very well defined and it is feasible to translate the conditions that are known to exist in a production pipeline to those used in laboratory tests [2, 3].
The working electrodes were machined out from commercial steel pipe grade API X65 and had wires attached to carry the electrochemical signals. The surface preparation was carried out by grinding/polishing with 1200 grit of silicon carbide paper then degreasing with iso-propanol to remove any dust and scratches so that only the corrosion examined is that which occurred in the cell. The three regions were then separated by carefully cutting along the boundaries and they were reassembled by mounting in epoxy resin. A Perspex plate was attached to the samples, separated by a 1 mm thick rubber gasket, to form a flow channel through which brine saturated with CO2 was pumped using a peristaltic pump see Figure 3.
Figure 3 Flow loop assembly
The areas of the exposed parent material (PM), heat affected zone (HAZ) and weld metal (WM) were in the ratio 8:1:2 to represent the approximate proportions of each region in the vicinity of a typical pipeline weld.
The wall shear stress, т channel, in the liquid at channel electrode surface was calculated from the following equation .
Where v is the flow velocity and / is the friction factor.
Electrochemical measurements, Galvanic Currents. The galvanic currents between each weld region were recorded every sixty seconds during the test using a multi-channel zero resistance ammeters (ACM Instruments GalvoGill 12) connected to a data logging personal computer. The currents from the parent material to the weld metal and from the HAZ to the weld metal were recorded on two channels and, as the three electrodes were in the short-circuit condition, their individual galvanic currents were established from the following relationship:
т =/ (V2 / 8 g)
IPM + IHAZ + IWM ~~ 0
Self-Corrosion Rates The self-corrosion rates of the three weld regions were measured by uncoupling each sample in turn and carrying out linear polarisation resistance measurements (LPR) using a platinum secondary electrode, a standard calomel reference electrode and a Sycopel Electronics AW2 low noise potentiostat. The potential of each weld component was scanned 10 mV above and below its open circuit value, at a scan rate of 10 mV min-1, and the current response was recorded. The polarisation resistance, Rp, was obtained from the best-fit gradient of the potential/current graph and the corrosion current, ICORR, was then found from the equation:
ICORR ~~ B / RP
Where B is a constant for the material and its environment, taken as 13 mV .
Experimental Conditions. The experiments were carried out at ambient temperature (20 C) in a cell containing artificial seawater, saturated with carbon dioxide for static and flowing conditions. However for flowing conditions, The flow channel was equipped with a fixed rubber hose for inlet and outlet of flow while, the glass cell (reservoir) filled at 0.8 litre capacity of 3.5% artificial sea water and fitted with the suitable cover lid with 3 inlets to allow for the CO2 injection as well as the discharge and suction of the solution electrolyte. Moreover, a peristaltic pump was used to maintain a uniform flow circulation and direction of 0.6 ms-1, which corresponded to a shear stress of 2.56 Pa.
In one experimental run, galvanic currents were recorded at 1 minute intervals through out the test, while in linear polarisation resistance measurements is carried out in the static and at flowing conditions. The same methodology was used to study the corrosion behaviour of each weld region in seawater in both the uninhibited condition and when containing 30 ppm by volume of a 'green' oilfield corrosion inhibitor (Clariant Oil Services).
Results and discussion, Galvanic Current Measurements (Uninhibited Conditions). The parent material and the HAZ were both anodic (+ve currents) to the weld metal at test temperature. The results for the ambient temperature condition are given in Figure 4. Clearly, it was favourable for the weld metal to be the cathodic component in the couple as this condition ensured that its corrosion rate was reduced by sacrificial protection of the other weld regions.
Galvanic currents increased approximately linearly during a period of ten days of exposure. It could be seen from the visual appearance that the flow caused partial removal of the film. Although the anodic current measured on the parent material was larger than that on the HAZ, when the different electrode areas were taken into consideration their current densities were very similar. This is not surprising as they had the same elemental composition and differed only in their microstructures.
Figure 4 Galvanic current densities under sweet corrosion (CO2) & flowing conditions for 10 days
Inhibited Conditions The galvanic currents that resulted from the addition of 30 ppm of oilfield corrosion inhibitor displayed different behaviour to those in uninhibited seawater and the results obtained in ambient conditions are shown in Figure 5. The presence of the inhibitor caused a marked change in the galvanic corrosion behaviour. A rapid current reversal took place with the weld metal becoming anodic to the parent metal and HAZ. The galvanic current was particularly high on the weld metal, suggesting that the inhibitor film was more adversely affected by flow on this material than on the other weld regions. It is thought that the high shear stress caused preferential removal of the inhibitor film from the weld metal, leading to an active shift in its potential. This situation is undesirable and would be expected to result in localised weld corrosion of the type that has sometimes been reported to occur in service .
Galvanic Test under Inhibited and Flowing Conditions
Time ( day )
Figure 5 Galvanic current density under inhibited and flowing conditions for 10 Days
Self-Corrosion rates (Uninhibited conditions). The self-corrosion rates, measured by LPR, displayed a steady increase over a period of 10 days, from approximately 0.3 mmy-1 to more than 4.5 mmy-1 with PM metal having slightly lower rate as shown in Figure 6.
LPR Test under Flowing & СОг Conditions
Time( Day )
Figure 6 Corrosion rates for uninhibited samples under flow conditions for 10 days
Inhibited condition. The self corrosion rate measurements of each weld section in inhibited solution were run for 10 days, as shown in Figure 7. Corrosion inhibitor had an effect on all three weld segments and a corrosion rate reduction was observed. It was observed that the green inhibiter was quite effective at 30 ppm. However, it was less effective for WM compared to other regions. It was observed that a black layer covered all three regions. In addition to having the highest self-corrosion rates, the HAZ and weld metal had been the anodes in the galvanic corrosion tests in inhibited conditions. However, as ranking the order of corrosion rates in the three weld regions it is unlikely that microstructural differences alone are the explanation. Rather, it appears that the cause was differences in surface protection related to inhibitor film formation, with poorer film properties occurring on the weld metal and HAZ than on the parent material.
LPR Test under Inhibited and Flowing Conditions |
Figure 7 Current density for inhibited samples under flow conditions for 10 days
CONCLUSION. 1. In uninhibited conditions the weld metal was cathodic to the parent material and HAZ. This reduced the total corrosion rate of the weld metal as it was partially protected by the sacrificial corrosion of the other two weld regions.
2. The addition of a 'green' oilfield corrosion inhibitor caused a current reversal under flowing conditions. The inhibitor film was removed preferentially from the weld metal, so that it became strongly anodic and lead to a condition that would result in severe localised weld corrosion.
3. It appears that preferential weld corrosion is caused by unstable conditions in which the inhibitor film is selectively disrupted on the weld metal but remains effective on the other weld regions.
1. G.I.Winning, N.Bretherton, A.McMahon & D.McNaughtan,(2004). Evaluation of weld corrosion behaviour and the application of corrosion inhibitors and combined scale/corrosion inhibitors, NACE Corrosion 2004, New Orleans, USA.
2. Gulbrandsen,E.and Dugstad, A.(2005), Corrosion Loop Studies Of Preferential Weld Corrosionand Its Inhibition In CO2 Environments', Corrosion, No 05276, Kjeller, Norway
3. Yang Yanga, Bruce Browna, Srdjan Nesic Maria Elena Gennarob, Bernardo Molinasb" (2011), Mechanical strength and removal of a protective iron carbonate layer formed on mild steel in CO2 corrosion", Paper No. 10383, Corrosion, NACE, Houston.
4. K. Alawadhi. (2010). Inhibition of Weld Corrosion in Flowing Brines Containing Carbon Dioxide. PhD Thesis, Cranfield University.
5. S.Nesic, J.Postlethwaite & S.Olsen, 91996). An electrochemical model for prediction of corrosion of mild steel in aqueous carbon dioxide solutions, Corrosion, 52, 280-294.
Целью настоящего научного исследования является исследование причины серьезной локализованной коррозии, которая иногда случается в сварных швах в трубопроводах из углеродистой стали, перемещающих углеводороды и препятствующие коррозии соляные растворы, насыщенные двуокисью углерода. Для моделирования реальных условий, которые происходят в трубопроводе, в лаборатории использовалась аппаратура проводящего канала. Различные образцы от основного металла, металла сварного шва и зоны термического влияния стального трубопровода, которые могли быть гальванически связаны, и испытывались при статических и текущих условиях с использованием и без ингибитора. Скорость течения полной коррозии каждой сварной зоны записывалась по сумме самокоррозии и гальванических вкладов. Дополнительная (вторичная) цель научной работы - исследовать, при каких условиях происходит обратный процесс. В работе использовались пять источников.
Ключевые слова: коррозия в сварных швах, углеродистая сталь, ингибитор, двуокись углерода, проводящий
Метою даної наукової роботи є дослідження причини інтенсивної локалізованої корозії, що іноді трапляється у зварних швах у трубопроводах з вуглецевої сталі, по яких транспортуються вуглеводні і соляні розчини, насичені двоокисом вуглецю, що перешкоджають корозії. Для моделювання реальних умов, які відбуваються в трубопроводі, у лабораторії використовувалась апаратура провідного каналу. Різні зразки від основного металу, металу зварного шва і зони термічного впливу сталевого трубопроводу, які могли бути гальванічно зв'язані, випробовувались при статичних і поточних умовах з використанням інгібітора та без інгібітора. Швидкість корозії кожної зварної зони записувалась за сумою самокорозії і гальванічних внесків. Додаткова (вторинна) ціль наукової праці - дослідити, за яких умов відбувається зворотний процес. У роботі використано п'ять джерел.
Ключові слова: корозія у зварних швах, вуглецева сталь, інгібітор, двоокис вуглецю, провідний канал.
Alawadhi K.A. Prof., dr.sci. College of Technological Studies, Auto.&Marine Eng. Dept., The Public Authority for Applied Education & Training, Kuwait.
A. Al-Ali* Prof., dr.sci. College of Technological Studies, Auto.&Marine Eng. Dept., The Public Authority for
Applied Education & Training, Kuwait.