Criterion for cathodic protection of 25CD4/Inconel 182 system
D. Allou
a,b,⁎, D. Miroud
b, M. Ouadah
c, B. Cheniti
a, S. Bouyegh
aaResearch Center in Industrial Technologies (CRTI), P.O.Box 64, Cheraga, 16014 Algiers, Algeria
bLaboratoire des Sciences et Génie des Matériaux (LSGM). USTHB, Algeria
cEcole Supérieureen Sciences Appliquées d’Alger, BP 474, Place des Martyrs Alger 16001, 1er Novembre, Algeria
A R T I C L E I N F O
Keywords:
Cathodic protection 25CD4
Inconel 182 Overlay
Cronopotentiometry
A B S T R A C T
This study aims to investigate the cathodic protection criterion of a galvanic system of low alloy steel 25CD4 substrate /Inconel 182 austenitic stainless-steelfiller metal couple obtained using Shield Metal Arc Welding (SMAW) process. The microstructure investigation revealed the presence of Type II boundary along the steel substrate/Inconel interface where high carbon content and high hardness were recorded. The electrochemical tests evaluated in marine environment (3.5% NaCl) at room temperature revealed that the corrosion potential (Ecorr) of the interface was between the steels substrate and the Inconel 182filler metal ones, On the other hand, the current density (Icorr) and corrosion rate were slightly higher in the overlaid area. In order to determine the system protection criterion, chronopotentiometry method was introduced. It was found that the cathodic pro- tection criterion for the assembly is based on the criterion of the least noble materialwhich is the steel substrate.
1. Introduction
Cathodic protection is the most important method of controlling corrosion[1].The protection against corrosion of all marine structures, including ships hulls and submarines requires the use of cathodic pro- tection which is the only reliable, efficient and economical method [2–4].Due to the industrial development that our planet is experien- cing, there is a great demand for new materials and combinations of different materials. The intersection of those different materials forms complex systems, hence the need to closely monitor the galvanic cor- rosion that takes place between these different systems[5–7].Galvanic corrosion is an electrochemical reaction that can be defined as the effect resulting from contact of two different metals or alloys in a conductive corrosive environment [8].Cathodic protection (CP) is one of the techniques the more used to prevent corrosion of a metal surface by making it the cathode of an electrochemical cell, via an impressed current anode or by connecting it to a sacrificial material assumed as a sacrificial anode (e.g. aluminium, zinc or magnesium)[9–11]. For this, there are two main cathodic protection system types, namely: im- pressed current cathodic protection and sacrificial anode cathodic protection as shown inFig. 1. The criteria for cathodic protection for a steel structure buried in the ground or submerged are determined by the“NACE”standard and defined in particular by the“SP0169″stan- dard (NACE SP0169, 2013)[12].For metallic structures buried in the soil for example the pipelines, the cathodic protection criterion is
−850 mV/Ag/AgCl[13]. On the other hand, for a complex system it becomes difficult to determine this criterion.
The aim of this paper is to determine the cathodic protection cri- terion of the 25CD4 steel substrate/Inconel 182 overlay system ob- tained by Shield Metal Arc Welding (SMAW) process. Microstructural examination is carried out to characterize the different zones of the corresponding system (i/e steel substrate and Inconel 182 overlay).
Electrochemical measurements of differentzoneswere also evaluated in a marine environment (3,5% NaCl) at room temperature. Furthermore, Chronopotentiometry method was used to determine the system pro- tection criterion using corrosion potential of the least noble material.
The results, indicate that the cathodic protection potential of the cou- pling is located in the cathodic protection zone of the steel substrate.
2. Materials and experimental procedures
The chemical composition of the materials used in this work is summarized inTable. 1. The deposited Metal employed in this work is Inconel 182, received in form of electrodes with 3.2 mm diameter and deposited on 25CD4 steel substrate surface (plat form of 25 mm thick- ness) using Shielded Metal Arc Welding (SMAW) process. Two overlay passes were conducted to have an extra thickness using a direct current (DC) mode with 170A and voltage of 33 V for thefirst pass and 220A and 34 V for the second pass.
Thereafter, all samples were cut and polished using standard
https://doi.org/10.1016/j.apsusc.2019.144100
Received 19 March 2019; Received in revised form 30 June 2019; Accepted 18 September 2019
⁎Corresponding author at: Research Center in Industrial Technologies (CRTI), P.O.Box 64, Cheraga, 16014 Algiers, Algeria.
E-mail address:djilallou@yahoo.fr(D. Allou).
Available online 15 October 2019
0169-4332/ © 2019 Elsevier B.V. All rights reserved.
T
metallographic techniques according to ASTM G1-03[14].The samples were then cleaned and chemically etched using Nital reagent (2% so- lution of HNO3 in ethanol) for 10 s. The microstructure was carried outand examined by optical microscope (OM, ZEISS). The mechanical tests were performed using Vickers hardness tester machine INNOVEST 9000 with 10 kg/f load and 1 mm distance between indents.
The electrochemical measurements have been conducted using a standard three-electrodes cell, in NaCl 3,5%electrolyte (35 g/L and pH of 7.8) at room temperature (24 ± 1 °C), prepared with analytical grade reagents and distilled water. The potential is referred to a Ag/
AgCl/KClsatelectrode (0.205 V vs SHE) and the counter electrode is in platinum, three samples with 1 cm2surface areas were used as working electrode: (i) steel substrate, (ii) Inconel 182 overlay and (iii) steel substrate/ Inconel 182 overlay interface (50% steel substrate and 50%
Inconel 182), respectively. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curve were measured by po- tentiostat PGSTAT302N, Autolab electrochemical analyzer instruments, Metrohm, with NOVA software. The EIS measurements were taken in a frequency range from 100 kHz to 100 MHz by applying 0.01 V ampli- tude. The potentiodynamic polarization curves were measured at with 0.01 V/s scan rate.
Electrochemical parameters such as corrosion potential Ecorrcorro- sion current (icorr) chemical parameters such as corrosion potential (Ecorr), anodic Tafel slopes (ba) and cathodic Tafel slopes (bc) were derived from the polarization curves by Tafel extrapolation[15]. After the stabilization of the open circuit potential (OCP), the cathodic po- larization curves were recorded dynamically at a scan rate of 5 mV/min from the OCP towards −1 V with respect to the OCP. The chemical stability and corrosion process of the working electrode during the immersion time can be followed by the variation of the open circuit potential (OCP). The sample was immersed into the solution for about 120 min to make its open circuit potential (Eocp) become stabilized. All potentials were measured relative to Ag/AgCl/KClsat reference elec- trode. All the measurements were repeated at least three times in order to check the repeatability of the experiments.
The chronopotentiometry have been used to follow the evolution of the potential as a function of time when the system is subjected to a current. It is an electrochemical technique which the current (i) is controlled and the potential is the variable determined according to the time E = f (t). This is a galvanic dissolution accelerated by the im- position of different current densities. This technique can be used to study the kinetics of the electrodes[16].
The galvanostat uses a three-electrode configuration in which a
current is applied between the working electrode and a platinum aux- iliary electrode, the potential of the working electrode (measured re- lative to the reference electrode (Ag / AgCl / KClsat) is monitored.
3. Results and discussion
3.1. Microstructural and hardness features
Fig. 2(a-c) showed the optical micrographs of the steel substrate, steel substrate/overlay interface and the interface between thefirst and the last pass of the overlay. It can be seen that the steel is mainly composed of ferrite and perlite structure where the amount of the fer- rite decreases with approaching the substrate/overly interface (Fig. 2b).
As shown in thisfigure, a formation of new narrow zone of about 10 µm along the substrate/overlay interface is observed. This zone, which is a characteristic of dissimilar metal joint, is known as Type II boundary, it is a solid state transformation that occurred during the cooling process and resulted in an austenitic grain growth. The literature attributed the formation of this area to the difference in the crystal structure between the materials to be assembled and reported it is lowchromium andhigh carbon content [25]. On the other side, the microstructure between passes (Fig. 2c) revealed the difference in the dendritic structure be- tween thefirst and the last pass where the dendrites of the later are finer then the former one and oriented following the cooling direction.
The hardness profile across steel substrate/ Inconel 182 overlay is shown inFig. 3. The steel substrate exhibits a low hardness compared to that of the Inconel 182 (150 HV10 and 210 HV10, respectively). A peak of hardness of about 290 HV10 is observednear the interface may be attributed to the presence of hard microstructure in the type II boundary that contains more carbon diffused from the carbon steel substrate toward the Inconel 182[17,18].
3.2. Electrochemical behavior
The evolution curves of open circuit potential (OCP) of the substrate (a), Inconel 182/Steel substrate interface(b) and Inconel 182 weld overlaid (c) according to time in the 3.5% NaCl solution are shown in Fig. 4. There is a decrease in the drop-out potential in thefirst few hours of immersion before reaching stability after 2 h. This decrease is related to the formation of a protective oxide layer on the surface of the ma- terial, which presumes a severity of the corrosive attack of the 3,5%
NaCl. The values of the free potentials, after stabilization, are shown in Table 3.
As shown inTable 3, the potential value of the substrate is smaller than that of the interface and the overlaid. Ecorr(Substrate) = -618 mV in 2h55min; Ecorr(Inconel 182/Steel substrate interface) =−88 mV in 2 h; Ecorr(Inconel 182 weld overlaid) =−139 mV in 1 min.
Electrochemical impedance spectroscopy (EIS) is commonly used to study the corrosion mechanisms of metals. The various parameters are acquired by simulation using equivalent electrical circuitsFig. 5. Each element of the electrical circuit represents a physical element of the Fig.1.Cathodic protection system. (a) Impressed current cathodic protection (b) Sacrificial anode cathodic protection.
Table 1
Chemical composition of the materials used.
Elements C Mo Si Mn Cr P S Ni Fe
25CD4 0,3 0,23 0,28 0,56 1,04 0,01 0,01 – Balance
Inconel 182 0.1 —— 0.4 8 15 0.03 0.015 Balance 5
metal/electrolyte interface. In particular the resistance of the electro- lyte (Re), the capacity of the double layer (Cdc), and the charge transfer resistance (Rt).
The plots of the impedance diagrams of the different zones, are shown inFig. 6(a,b,c).
One loop is observed for each area and reflects the same chemical process, namely the active dissolution of the metal. The value of the total resistance (56.6 KΩ/cm2) of the Inconel 182 weld overlaid, ob- tained by simulation is high compared to that obtained in the substrate and Inconel 182/Steel substrate interface, which confirms the lower corrosion rate.
The electrochemical impedance spectroscopic of the different zones (substrate, Inconel 182 weld overlaid and Inconel 182/Steel substrate interface) in 3.5% NaCl solution is recorded in the frequency range from 100 kHz to 0.01 Hz using 10 mV peak-to-peak signal amplitude.
The plot of the imaginary parts of impedance against the real parts give the electrochemical impedance spectroscopy plots, as shown in Fig. 5(a,b,c).
The electrical equivalent circuit of the different zones in the point of view to the electrochemistry of the metal/electrolyte interface is pre- sented inFig. 6, where Rsis the solution resistance, C is the double- layer capacitance and Rpis the charge-transfer resistance (Rpis pro- portional to the corrosion resistance of the electrode. With the increase of Rp, the resistance to the corrosion increases.)
The results offitting the electrochemical impedance spectroscopy curves fromFig. 5(a,b,c) are summarized inTable 2which indicate the solution resistance (Rs), the charge-transfer resistance (Rp) and the double-layer capacitance (C). From Fig. 5, it can be seen that the charge-transfer resistance of the Inconel 182 weld overlaid is greater than the Inconel 182/Steel substrate interface and the substrate. This means that the corrosion rate of the Inconel 182 is lower compared the substrate and the interface.
The polarization curve is the basic kinetic law for any electro- chemical reaction. It is a plot of logarithm of the current density log (I) versus electrode potential for a specific electrode- electrolyte combi- nation.Fig. 7.shows the superposition of the polarization curves of the system (substrate, Inconel 182/Steel substrate interface and Inconel 182 weld overlaid) in NaCl (3,5%) obtained from scanning range be- tween −1 and 0.1 V. Ag/AgCl reference electrodes for the samples substrate and interface, and between −0.6 and 0.4 V. Ag/AgCl re- ference electrode for the sample weld overlaid. We used the scan rate of 1 mV/s from the cathode to anode direction.
The results offitting the polarization curve from Fig. 7.are sum- marized inTable 3, which indicate the corrosion potential (Ecorr) and the corrosion current density (icorr). It can be seen that the corrosion potential of the substrate (a) is−668 mV. Ag/AgCl, and the corrosion potential of the Inconel 182 weldoverlaid is −0.235 mV.Ag/AgCl.- Concerning the Inconel 182/Steel substrate interface (b), it can be seen that their corrosion potential is 626 mV.Ag/AgCl witch representing the intersection between the cathode branch of the substrate and the anodic
overlaid
1
stpass
2
ndpass
Inconel 182 weld overlayaid
c
Fig. 2.Microstructure of, (a) substrate, (b)substrate/Inconel182 interface c) Interface between 1st pass and 2nd pass.
-10 -8 -6 -4 -2 0 2 4 6 8 10 12
140 160 180 200 220 240 260 280 300
Vickers Hardness HV10 Fusion line
Distance from fusion line (mm)
Inconel 182 deposited metal 25CD4 substrat
Fig. 3.Hardness surveys across the Inconel 182/substrate interface.
branch of the Inconel 182 weld overlaid. Also, the least noble material of the coupling between low alloy steel substrateand Inconel 182 aus- tenitic stainless-steel metalweld overlaid is the steel 25CD4 substrate.
The appearance of pits on the surface of substrate and inconel182 weld overlaid is attributed to the attack of chloride ions on the passive film, mainly Cr2O3,on Inconel 182 weld overlaid[19].
It can be noticed that the corrosion rate follows the logic such that the substrate (0143 mm/year) is greater than that of the Inconel 182/
Steel substrate interface (0115 mm/year) and the Inconel 182 weld overlaid (0012 mm/year) respectively.
The results obtained by the electrochemical impedance spectro- scopic are confirmed by the polarization curves. From the result, it is clearly seen that the corrosion current of the Inconel 182 deposited
metal (1137 mA/cm2) is lower than the corrosion current of the Inconel 182/Steel substrate interface (537 mA/cm2) and the 25CD4 substrate (18,53 mA/cm2). Concerning the corrosion potential, it can be seen that the corrosion potential of the substrate is more negative than the cor- rosion potential of the Inconel 182/Steel substrate interface and the Inconel 182 deposited metal. From an electrical point of view, this re- sult means that the system forms two corrosion cells. Thefirst cell is formed between the substrate (anode) and the Inconel 182/Steel sub- strate interface (cathode), the second cell is formed between the Inconel 182/Steel substrate interface (anode) and the Inconel 182 deposited metal (cathode). To mitigate all corrosion including the microscopic corrosion cells on the surface of the anodic metal, the metal couple must be cathodically polarized. The question that arises, which cri- terion should be used for a given structure and what is the basis of its selection. Although there is a very often mention in the literature to
−850 V as a criterion for protection of steel pipe and to its origin, the exact origin of the criterion requiring a change of potential of 300 mill volts is obscure.
3.3. Cathodic protection criterion of the system
The cathodic protection potential criterion is based on soil en- vironment at ambient temperatures. This criterion has decreasing va- lidity as temperature increases or unusual conditions and chemistries are encountered. Some of the factors that can affect the validity and application of specific criteria include temperature, alternating current interference, and presence of mixed metals (galvanic coupling between Fig. 4.Open circuit potential (OCP) curves (a) Substrate, (b) Interface and (c) Deposited metal.
Fig. 5.Equivalent electrical circuit.
metals) [20–23].According to NACE International[24], the cathodic protection criterion of a galvanic coupling between two materials is based on the protection criterion of the least noble material. In this part of the study, we investigate the cathodic protection criterion of a system consisting of a galvanic coupling between a low alloy steel 25CD4 /Inconel 182 austenitic stainless-steelfiller metal. To achieve this ob- jective,firstly, we proceed to determine the cathodic protection current of the least noble material (25 CD4 substrate) using Chron- opotentiometry method. Then, we will apply the obtained protection current to our galvanic coupling 25CD4/Inconel 182 austenitic stain- less-steel filler metal, to see the variation of the cathodic protection potential of the coupling25CD4/Inconel 182. The criterion for complete cathodic protection of the 25 CD4 substrate is a negative potential of at least 850 mV with the cathodic protection applied. For a cathodic
protection voltage of −850 mV.Ag/AgCl reference electrode, Fig. 8 shows the cathodic protection current of the substrate as a function of the time. From thisfigure, it can be seen afluctuation of the cathodic protection current of the steel 25 CD4 around 20μA. Thisfluctuation is due to the stability of the electrochemical measurement system. After the determination of the cathodic protection current of the steel sub- strate, the obtained protection current is applied on our galvanic cou- pling system (25CD4 steel substrate/Inconel 182 austenitic stainless- steelfiller metal).Fig. 9.shows the cathodic protection potential profile as a function of the tine of the coupling system. As it is shown inFig. 7, when substrate coupled to a more cathodic metal Inconel 182 the corrosion potential of the couple (substrate /Inconel 182) was shifted electropositively. This potential shift is clearly remarkable when the protection current of the steel substrate is applied to the galvanic coupling substrate/Inconel 182 austenitic stainless-steel filler metal.
FromFig. 9, it can be seen that the cathodic protection potential of the coupling is located in the cathodic protection zone of the steel. This means that the galvanic coupling between the low alloy steel substrate/
Inconel 182 austenitic stainless-steel filler metal is protected from corrosion.
4. Conclusions
Thecathodic protection criterion of a system consisting of a galvanic coupling between low alloy steel 25CD4/Inconel 182 austenitic stain- less-steelfiller metal,using Shield Metal Arc Welding (SMAW) process, have been studied:
•
The hardness of the interface next to the substrate is high caused by chemical composition different between the substrate and thefiller metal and higher carbon content in substrate.•
One loop is observed for each area. The plots of the impedance diagrams reflects the same chemical process, namely the active Fig. 6.Electrochemical Impedance Spectroscopy (EIS) curves; (a) Substrate, (b) Interface and (c) Deposited metal.Table 2
Electrochemical Impedance Spectroscopy (EIS) measurements.
Elements Substrate Interface
25CD4/Inconel 182
Weld overlaid
Rs(Ω) 8.03 10,07 3,86
Rp (KΩ) 1,54 1,67 56,60
C (µF) 1,51 1,67 52
Table 3
Electrochemical parameters of potentiodynamic tests.
Elements Substrate Interface
25CD4/Inconel 182
Weld overlaid
Ecorr(mV) −668 −626 −235
Time(s) 9200 7200 48
Icorr(mA/cm2) 18,53 5,37 1,137
Corrosion rate (mm/year) 0,143 0,115 0,012
dissolution of the metal.
•
The corrosion potential of the interface is the intersection between the cathode branch of the substrate and the anodic branch of the overlaid.•
The corrosion current density of the galvanic coupling (a low alloy steel 25CD4/Inconel 182 austenitic stainless-steel) is situated be- tween the corrosion current densities of a low alloy steel 25CD4 andInconel 182 austenitic stainless-steel.
•
Thecathodic protection criterion for thegalvanic coupling (a low alloy steel 25CD4/Inconel 182 austenitic stainless-steel) is based on the cathodic protection criterion of the low alloy steel 25CD4.E (V / Ag/AgCl/KCl
sat) Ag/AgCl/KClsat I(A/cm
2)
Interface
Weld overlaid
Fig. 7.Superposition of potentiodynamiccurvesof Substrate, Interface and weld overlaid.
Fig. 8.Protection current density of the substrate material.
Fig. 9.Cathodic protection potential of the galvanic coupling substrate/Inconel182 deposited metal.
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