Laboratory investigation of cathodic protection of ASTM A36 carbon steel in seawater under alternative potentials

Abstract

Cathodic protection is a technique used to prevent corrosion of structures in seawater. In the present work, cathodic protection of ASTM A36 carbon steel was investigated under two polarization conditions in synthetic and natural seawater, at 25°C and 4°C. Open circuit potential (OCP) of the material in seawater was approximately −700 mV_Ag/AgCl enabling investigation of an alternative protection potential of −750 mV_Ag/AgCl compared to the standard value of −800 mV_Ag/AgCl. Laboratory polarization experiments were complemented by thermodynamic equilibrium simulations. Results demonstrated that steel integrity is preserved at the alternative potential, with uniform corrosion rate decreasing at lower temperatures. This study advances the understanding of cathodic protection in subsea systems by integrating kinetic and thermodynamic criteria, while highlighting the role of temperature directly linked to installation depth. This analysis contributed to improving knowledge on cathodic protection mechanisms in subsea systems by integrating kinetic considerations into existing thermodynamic criteria, ensuring minimal risk to structural integrity. These findings pave the way for reducing the need for submarine interventions to restore cathodic protection systems and facilitate maintenance of electrode potentials slightly above the limits set by standards.

Keywords

corrosion cathodic protection carbon steel alternative potential

Introduction

Seawater is a complex environment in which parameters such as salinity, temperature, oxygen content, and pH vary according to depth, season, and geographic location. These variations can significantly influence the corrosion of materials exposed to the marine environment and the corrosion simulation in seawater is essential to understanding and mitigating damages. In laboratory simulations, synthetic seawater solution standardized by ASTM D1141¹ is commonly used due to its chemical and physicochemical similarity to natural seawater.^2,3

When structural metals like carbon steel are exposed to this environment, corrosion protection becomes essential. Among the available techniques, cathodic protection and protective coatings are the most widely used. Cathodic protection (CP) is an electrochemical method that reduces or prevents corrosion by applying an electric current to the metal, shifting its potential to more negative values, referred as cathodic potentials.^4,5 This shift would put the metal into the immunity region in the Pourbaix diagram of iron (Fe), the main component of carbon steel. In this thermodynamically stable state, corrosion could not occur, and the corrosion rate would be theoretically zero.⁶

Consequently, cathodic protection standards criteria for carbon steel structures state that when a potential of −800 mV relative to the silver-silver chloride (Ag/AgCl) reference electrode is reached, corrosion protection is guaranteed, regardless of depth. At this potential, the metal would be within the immunity domain for seawater pH, approximately 8.00, halting the corrosion process.^4,^7–9 However, although the −800mV_Ag/AgCl criterion is accepted and widely used, it cannot be taken as an absolute value. According to electrochemical thermodynamics, this potential represents an equilibrium condition that can be influenced by environmental variables such as temperature, pressure, and especially the concentration of ferrous ions present in seawater.¹⁰ The adoption of an absolute cathodic potential value is also discussed by Googan,¹¹ who reports that, considering purely equilibrium thermodynamic principles, the achievement of immunity would require electrochemical potentials more negative than −1060mV_Ag/AgCl, based on ferrous ion concentrations expected in seawater. However, this author claims that such values are impractical in applications and would be incompatible with the capacity of conventional galvanic polarization systems. Furthermore, the author emphasizes that thermodynamic criteria are insufficient to interpretate the mechanism of protection, as they disregard kinetic factors, which are effective to provide corrosion protection in practice.

Another important factor in this corrosion process in seawater refers to the influence of temperature. The location of subsea equipment in deeper water depths leads to the consideration of different temperatures, variable oxygen contents and other factors that directly interfere with the corrosion process of carbon steel in seawater environment and the conditions required for its protection. According to Melchers,¹² the temperature variation with depth is a significant factor, and data from a study conducted in Sargasso Sea demonstrated that the corrosion rate is directly correlated with the seawater temperature. In deep waters, the temperature is lower, and a reduction in corrosion rates usually occurs. This performance was reaffirmed in the study by Rocha et al.,¹³ who analyzed underwater inspection reports from Petrobras and found that, in lower temperature sites, there were no significant differences between corrosion diagnostics issued for subsea installations working under standard cathodic protection conditions and those operating under non-normalized protection range at certain depths.

Temperature influences several processes such as corrosion kinetics, oxygen concentration, ion diffusion, and biological activity, including the activity of sulfate – reducing bacteria (SRB), which can play a significant role in corrosion. These factors also have an influence on the formation of potentially protective deposit layers, such as CaCO₃. The formation and effectiveness of calcareous layer as a protective barrier against corrosion can be affected by temperature, where higher temperatures can accelerate the initial formation process of this layer, but can also have a negative influence on its long-term stability.²^,^14–16

According to the technical literature, the formation of a calcium/magnesian layer is often observed in cathodically polarized carbon steel structures in seawater. This formation comes from the salts dissolved there. Bicarbonate, when under alkaline conditions generated by the cathodic reaction, tends to transform into carbonate and hydroxide, forming precipitates of calcium carbonate (CaCO₃) and magnesium hydroxide (Mg (OH)₂), according to Equations (1)–(4).⁶ These formations are a consequence of the reduction of oxygen on the metal surface, under cathode potentials, which generates hydroxyl and increased pH at the interface.

H C O_{3}^{-} + O H^{-} \to H_{2} O + C O_{3}^{2 -}

(1)

M g^{2 +} + 2 O H^{-} \to M g (O H)_{2}

(2)

C a^{2 +} + C O_{3}^{2 -} \to C a C O_{3}

(3)

C a^{2 +} + M g^{2 +} + 2 C O_{3}^{2 -} \to C a C O_{3} . M g C O_{3}

(4)

These precipitates form a surface layer, locally reducing the dissolved oxygen content and acting as a physical barrier that additionally protects against corrosion. Consequently, the current density demand to maintain a given cathodic protection potential tends to be lower over time in seawater.^15,17

Given these observations, Googan¹¹ suggests that applying potentials above the minimum prescribed by standards could be viable, since a gap in the literature still remains regarding relevant corrosion damage at potentials between −700 and −800 mV. The author notes that in the late 1960s, the British Royal Navy successfully employed an alternative potential of −750 mV for its warships. Similarly, Moore and Knuckey proposed −770 mV as a practical value that, while outside standardized norms, provides an economically viable level of protection. These researchers argue that cathodic protection should not be viewed as requiring a potential that completely prevents corrosion, but rather one that reduces corrosion rates to acceptable levels. This perspective suggests that achieving −800 mV may not represent the most effective approach, and that the focus should instead be on reducing corrosion rates rather than achieving absolute protection.^10,16

In this context, the objective of this study was to evaluate the corrosion rate of ASTM A36 carbon steel under attenuated cathodic protection in both synthetic and natural seawater prepared according to ASTM D1141¹ standard. The applied potentials were −750 and −810 mV_Ag/AgCl using potentiostatic control method.

The tests were initially conducted in synthetic seawater at 25 °C. Subsequently, to investigate the effects of temperature, experiments were conducted in synthetic seawater at 4 °C, simulating the conditions at higher seawater depths. Later, comparative tests were performed in natural seawater at the same temperature to assess possible differences between the two environments – natural and synthetic.

Potentiostatic control was adopted to permit a direct correlation between the imposed cathodic potential and the corrosion rate. The open circuit potential was initially measured. Electrochemical techniques of linear polarization resistance (LPR) measurements and cathodic and anodic polarization curves were used to estimate the corrosion rates in the cathodically unprotected material. Mass loss tests (gravimetric) were used to measure the corrosion rates under cathodic polarization and to validate the electrochemical measurements done at OCP. After the tests, surface analyses were carried out to evaluate the corrosion morphology and to identify possible deposit formation on the ASTM A36 steel.

Experimental section

Digital simulation

The experimental conditions selected were used to simulate thermodynamic E.pH equilibrium diagrams to assess the influence of the environmental conditions on the immunity and corrosion domains of iron in seawater. The software used was HSC10 installed in a workstation.

Material characterization and preparation

The samples were machined from a rolled ASTM A36 steel plate, whose chemical composition is shown in Table 1.

Table 1.

Chemical composition of ASTM A36 carbon steel.

Material	Chemical Composition (wt%)
	C	Mn	Si	P	S	Cr	Ni	V	Al	Cu	Ti	Sn
ASTM A36	0.09	0.41	0.01	0.016	0.009	0.01	0.01	0.001	0.044	0.01	0.001	0.001

Before the corrosion tests, the samples, coupons and electrodes, wet ground up to grid 600 using SiC emery paper, rinsed with distilled water, ethanol and dried with hot air to the removal of impurities and residual moisture. Liquid tape coating was applied to limit the approximately 2.30 cm² area exposed to the environment.

Test environments and conditions

Synthetic and natural seawater were used as electrolytes for corrosion tests carried out for 30 days. The synthetic seawater was used in tests conducted at 25°C and 4°C, prepared according to the parameters established by ASTM D1141.¹ The composition of the solution is found in Table 2. This standard provides guidelines to produce synthetic seawater that replicates the characteristics of natural seawater. To adjust the pH of the solution to 8.2, a 0.1 M NaOH solution was added. Natural seawater was used in tests carried out at 4°C. The test conditions were:

Condition A: synthetic seawater at 25°C, pressure of 1 bar and without agitation.

Condition B: synthetic seawater at 4°C, pressure of 1 bar and without agitation.

Condition C: natural seawater at 4°C, pressure of 1 bar and without agitation.

Table 2.

Constituents of synthetic seawater. Adapted from ASTM D1141.¹

Component	Concentration (g/L)	Component	Concentration (g/L)
$N a C l$	24.534	$N a H C O_{3}$	20.1
$N a_{2} S O_{4}$	4.094	$K B r$	10.0
$M g C l_{2}$ . $6 H_{2} O$	555.6	$S r C l_{2} .6 H_{2} O$	2.1
$C a C l_{2}$ (anidro)	57.9	$N a F$	0.3
$K C l$	69.5	$H_{3} B O_{3}$	2.7

Physicochemical analyses were performed in natural and synthetic seawater, including measurements of pH, dissolved oxygen (DO) and alkalinity, to characterize the initial conditions of the solution.

Experimental setup and procedure

The experiments were conducted at room temperature, 25°C, and at a lower temperature, 4°C. Pressure was 1 atm and natural aeration was maintained. The setup used is shown in Figure 1. For the low-temperature tests, a Lauda cryostat bath model RP240E was used.

Figure 1.

Diagram of the electrochemical cells and instruments used to perform the corrosion tests. (a) traditional three-electrode electrochemical cell, consisting of counter electrode (CE), reference electrode (RE), and working electrode (WE), used for tests at 25 °C; (b) three-electrode electrochemical cell coupled to a cryostat bath for temperature control, used for tests at 4 °C.

The electrochemical measurements were performed following a standard three electrode cell configuration, consisting of the steel sample as a working electrode, reference electrode of Ag/AgCl (3 M KCl) and Pt wire was used as counter electrode. The volume/area ratio was 217 mL/cm². The potentiostat used was a multichannel Palmsens 4 and the measurements made were open circuit potential (OCP), LPR, cathodic and anodic polarization curves and chronoamperometry. Each test was conducted three times.

OCP was measured over time during the test, in order to understand the behavior of the potential at the interface between the metal and the solution. Through the OCP values, it was possible to verify the evolution of the carbon steel potential in synthetic seawater and confirm the cathodic characteristics of the potentials to be applied by the CPS polarization.

LPR technique was used to follow up the evolution of the corrosion rate of carbon steel. In addition, it is a non-destructive technique that allows you to monitor the polarization resistance (Rp) of carbon steel throughout the test. The LPR technique was applied together with the OCP technique on the same electrodes. After stabilization of the OCP, an overvoltage of ±15 mV was applied using 0.33 mV/s scan every 12 h. This allowed to obtain polarization resistance (Rp) values by means of linear adjustment of the LPR graphs. The parameters follow the guidelines of ASTM G102-89¹⁸ and previous studies.

Cathodic and anodic polarization curves were performed by applying a cathodic overvoltage of ±300 mV in relation to corrosion potential (Ecorr). First, the electrode was cathodically polarized and the potential stabilization was waited before initiating anodic polarization. Current limitation of 500 μA/cm² (0.5 mA/cm²) was defined to avoid excessive degradation of the polarized sample.

Based on the values of the polarization resistance, determined by the inclination of the linear adjustment and the parameter B, according to the Stern-Geary (Equation 5), it was possible to calculate the corrosion current density. Then, using the Faraday Law, the instantaneous corrosion rate for ASTM A36 steel was determined as prescribed by the LPR method.^18,19

B = \frac{β_{a} . | β_{c} |}{2.303 . (β_{a} + | β_{c} |)}

(5)

Chronoamperometry was used to record the applied current at the −750 mV and −810 mV potential throughout the 30 days immersion period. Current density was continuously monitored in relation to time, allowing the evaluation of the electrochemical response of carbon steel under these specific conditions.

Mass loss test was conducted in accordance with ASTMG1-03 and NACE TM0169/G31.^20,21 The A36 coupons were carefully weighed on a high-precision analytical balance before and after 30 days immersion in seawater solution, and mass lost was calculated. For the weight loss measurements after corrosion tests, the samples were pickled in a Clark solution for 3 min and then immersed in distilled water to eliminate any remaining chemical residues, in accordance with ASTMG1-03.²⁰ To ensure complete cleaning of the metal surface, the coupons were subjected to an acetone bath and treated with an ultrasound.

Results and discussion

Digital simulation – thermodynamic E-pH diagrams of iron in seawater

Thermodynamic simulations based on Pourbaix diagrams are commonly used to identify the corrosion, passivity, and immunity domains of metals as a function of electrochemical potential and pH. The set of thermodynamic constants used as input data for these simulations is summarized in Table 3, ensuring reproducibility of the results.

Table 3.

Thermodynamic constants used as input dataset for HSC10 simulations.

Dielectric Constant: 78.38			ΔG of H₂O (kcal/mol): −56.68
Species	ΔG(kcal/mol)
	4°C		25°C
	[10⁻⁶] x mol/Kg H₂O	[10 −10] x mol/Kg H₂O	[10⁻⁶] x mol/Kg H₂O	[10 −10] x mol/Kg H₂O
Fe	0.00		0.00
FeO	−58.71		−58.36
Fe₂O₃	−178.92		−177.54
Fe₂O₃(E)	−172.87		−171.54
Fe₂O₃(G)	−176.14		−174.79
Fe₂O₃(H)	−160.81		−159.43
Fe₃O₄	−243.70		−241.96
Fe₃O₄(H)	−205.43		−203.86
Fe(OH)₂	−117.77		−116.39
Fe(OH)₃	−170.23		−168.09
Fe₂O₃*H₂O	−235.63		−233.23
Fe⁽⁺²⁾	−19.02		−18.85
Fe⁽⁺³⁾	−4.64		−4.11
FeOH⁽⁺¹⁾	−66.71		−65.85
FeOH⁽⁺²⁾	−58.67		−57.83
Fe(OH)₂⁽⁺¹⁾	−109.67		−108.08
Fe₂₍OH)₂⁽⁺⁴⁾	−114.21		−111.74
HFeO₂⁽⁻¹⁾	−97.49		−95.35

In the case of iron, the diagram generated under standard conditions — 25 °C, 1 bar, considering the ferrous ion concentration of 10⁻⁶ mol/L — shows that, at pH 8, the transition from corrosion to immunity domain occurs at approximately −800 mV_Ag/AgCl (E1 in Figure 2(a)), a value widely adopted by cathodic protection standards. However, simulations carried out using HSC10 software showed that changes in environmental parameters can significantly alter the potential required to achieve immunity. When the ferrous ion concentration is reduced to 10⁻¹⁰ mol/L, the protection potential for carbon steel shifts to approximately −910 mV_Ag/AgCl (E2 in Figure 2(a)). This result suggests that, under other conditions, the standard value of −800 mV_Ag/AgCl may not be sufficient to ensure full protection against corrosion, confirming the findings of Googan,¹¹ who highlights the influence of low ionic concentration environments and variable electrochemical conditions.

Figure 2.

Regions of interest in the E–pH diagram of the Fe–H₂O system simulated using HSC software. (a) Simulation at 25 °C and 1 bar pressure, with a ferrous ion concentration of 10⁻⁶ mol/L, immunity region at approximately −800 mV_Ag/AgCl (E1). Ferrous ion concentration 10⁻¹⁰ mol/L, immunity region shifts to approximately −910 mV_Ag/AgCl (E2). (b) E–pH diagram of the Fe–H₂O at 4 °C and 1 bar, and ferrous ion concentration of 10⁻⁶ mol/L, iron enters the immunity region at approximately −780 mV_Ag/AgCl (E3). When the concentration is further reduced to 10⁻¹⁰ mol/L, the protection potential shifts to approximately −890 mV_Ag/AgCl (E4).

It is worth noting that the inclusion of Fe₂O₃ and Fe₃O₄ phases in the equilibrium diagrams should be interpreted as a thermodynamic simplification. In marine environments, the predominant Fe (III) products are iron oxyhydroxides, mainly goethite (α-FeOOH) and lepidocrocite (γ-FeOOH), formed through intermediate green rusts I and II as established by Detournay et al., Misawa et al. and Refait et al.^22–25 As for Fe (II), although green rusts are the most representative phases under abiotic conditions, magnetite (Fe₃O₄) is also found in chloride media, particularly in locally reducing regions or as a transformation product of green rusts. Due to its high thermodynamic stability (µ° = 242,700 cal/mol), Fe₃O₄ is conventionally used in E–pH diagrams as a representative Fe (II) phase. These diagrams, however, describe only equilibrium conditions and not the actual kinetic sequence of iron oxidation. Thus, their use here serves as a practical approach to define stability domains, without contradicting the experimental evidence that points to green rusts and FeOOH phases as the main corrosion products in seawater.

Additional simulations were conducted considering variable temperature. Considering the ferrous ion concentration of 10⁻⁶ mol/L, lowering the temperature to 4 °C caused the protection potential to shift to −780 mV_Ag/AgCl (E3 in Figure 2(b)). For the lower concentration (10⁻¹⁰ mol/L), the adjusted potential was −890 mV_Ag/AgCl (E4 in Figure 2(b)). These results indicate that, at lower temperatures, lower cathodic overpotential would be required to achieve immunity. It becomes clear that both temperature and ionic concentration are relevant variables that influence the thermodynamic protection potential. It is important to note that, although pressure variations were not considered in these simulations, previous studies indicate that, under the conditions analyzed, pressure does not significantly affect the results when compared to temperature and ion concentration.²⁶

Thus, the gap between the theoretical protection potential and the standardized value commonly used in industry raises important questions regarding the scientific basis of the −800 mV_Ag/AgCl criterion and its applicability in different scenarios. As already noted by Googan,¹¹ real-world environments are far from ideal and are often characterized by low ferrous ion concentrations and dynamic changes in electrochemical conditions. In such cases, the adoption of a fixed protection potential based solely on equilibrium thermodynamics may be insufficient. The effectiveness of cathodic protection should therefore be assessed with an emphasis on reducing corrosion rates over time, also considering kinetic aspects, species transport, and the operational feasibility of the protection system.

The application of cathodic protection in seawater can increase the pH at the steel/solution interface up to approximately 9.50, which at very negative potentials (< −900 mV_Ag/AgCl) may favor the formation of calcareous deposits. However, in this study, less negative potentials (−750 and −810 mV_Ag/AgCl) were applied, under which no evidence of such layer formation was observed. It should be noted that a pH value of 8 was adopted as a reference, as it is a well-known value in the literature and specified by the ASTM D1141 standard for synthetic seawater. In addition, the physico-chemical parameters were measured within the bulk solution, and in the following section the actual pH values obtained using this methodology will be presented. Therefore, the use of pH = 8.00 as a reference does not compromise the conclusions of the study and is consistent with both the normative and experimental conditions.

In addition, the activity of sulfate-reducing bacteria (SRB) is often relevant in natural marine environments, as it may lead to FeS formation and require a more negative protection potential (around −900 mV_Ag/AgCl).^8,9 In the present study, however, this criterion was not applied. Metagenomic analysis of the seawater used, reported by Correa C. et al.,²⁷ showed negligible SRB abundance (≤0.73%). The samples were collected offshore, approximately 200 miles from the Brazilian coast, in an environment not affected by anthropogenic influence.

Table 4 provides the values of these transition lines obtained in the thermodynamic simulation by the HSC program in detail, considering a pH value 8.00.

Table 4.

Values of the Fe/Fe²⁺ line obtained from the E-pH diagram of the Fe-H₂O considering different concentrations of iron ions and temperature.

Simulated conditionpH = 8.00	Potencial (mV_Ag/AgCl)
Simulated conditionpH = 8.00	Fe/Fe²⁺ line
[10⁻⁶] M, 25°C	−800
[10⁻¹⁰] M, 25°C	−910
[10⁻⁶] M, 4°C	−780
[10⁻¹⁰] M, 4°C	−890

Electrochemical corrosion experiments

The initial pH of both synthetic seawater (according to ASTMD1141¹) and natural seawater was approximately 8.00. Throughout the experiments, pH was stable, as detailed in Table 5. Dissolved oxygen concentration varied slightly over time, registering a decrease of approximately 0.17 ppm in relation to the initial value. On the other hand, for conditions B and C, an increase in DO concentration of about 2 ppm was observed. This variation can be attributed to higher oxygen solubility at lower temperatures (Table 5).

Table 5.

Data from physicochemical analyzes of A36 carbon in each condition.

OCP
Condition	Time (day)	pH	DO (ppm)
A	0	8.00	7.53
	30	7.78 ± 0.07	7.36 ± 0.08
B	0	7.93	8.42
	30	7.63 ± 0.12	10.89 ± 0.67
C	0	7.94	8.11
	30	7.92 ± 0.15	10.59 ± 0.36

Figure 3 presents the OCP evolution under each condition, with the curves representing triplicate measurements. Under Condition A (ASTM D1141 – 25°C), the potentials fluctuated around −725 mV_Ag/AgCl during the first 200 h, gradually decreasing and stabilizing at −706 mV_Ag/AgCl after 360 h of immersion. In Condition B (ASTM D1141 – 4°C), the potentials oscillated around −700 mV_Ag/AgCl for the first 216 h before stabilizing at −687 mV_Ag/AgCl after 280 h. Meanwhile, in Condition C (Natural – 4°C), the potential stabilized at −696 mV_Ag/AgCl.

Figure 3.

Variation of the open circuit potential (OCP) over the immersion time of A36 carbon steel in each condition. Condition A: ASTMD1141 at 25°C, Condition B: ASTMD1141 at 4°C and Condition C: natural at 4°C.

The literature suggests that results obtained from synthetic and natural seawater tests exhibit similar trends, as observed in Conditions B and C. Xu et al.³ reported that EH 36 carbon steel displayed comparable behavior, with a potential of −727 mV_Ag/AgCl after 24 h under similar conditions, evidencing the transition from a transient to the stationary over time.

Additionally, studies by Kim et al.²⁸ observed an open-circuit potential (OCP) of approximately −686 mV_Ag/AgCl in synthetic seawater at 25°C under static conditions. Likewise, Larché et al.²⁹ reported an OCP of −673 mV_Ag/AgCl in natural seawater, considering a low flow rate (< 0.2 m/s), which was nearly stagnant.

After reaching the steady state, it is important to note that the final open-circuit potential values remained higher than the protection potentials, standard and alternative, which are highlighted by the region delineated in Figure 3.

Based on polarization curves, anodic (βanodic) and cathodic (βcathodic) constants were determined through the extrapolation of Tafel lines, enabling the calculation of the Stern-Geary coefficient (B), as shown in Equation 5. Although the technical literature provides approximate values for this coefficient — typically ranging from 13 to 26 mV—the use of tabulated values without considering the specific characteristics of the system, especially variations in electrolyte type and temperature, can introduce significant errors in the estimation of the corrosion rate. Therefore, it is recommended that the B coefficient be determined experimentally, through polarization curves and Tafel line extrapolation whenever possible. This approach allows for the determination of B values that are more representative of each specific condition, contributing to greater accuracy.

In this study, the B values were experimentally determined based on variations in parameters such as the type of electrolyte (natural or synthetic seawater) and the temperature (25 °C or 4 °C). The coefficients obtained were: 8.46 mV for Condition A, 12.94 mV for Condition B, and 11.80 mV for Condition C. This variation reflects differences in the anodic and cathodic behavior of carbon steel as a function of the environmental conditions. Although higher B values are commonly reported in the literature (typically between 13 and 26 mV), the lower values obtained in this study cannot be considered a discrepancy. Rather, they are consistent with the specific conditions analyzed, where temperature and oxygen transport strongly influenced corrosion kinetics and the control of the cathodic reaction in seawater. Therefore, the experimental determination of B is essential, as real scenarios often diverge from theoretical estimations and standard expectations.

The polarization electrochemical resistance of the material was determined using the Linear Polarization (LPR) technique, as specified in ASTM G59-97.¹⁹ Based on the values and the corrosion current density, it was possible to calculate the instantaneous corrosion rate over the 720-h test period, following the guidelines of ASTM G102-89.¹⁸ Figure 4 presents the representative curves of the variation in instantaneous corrosion rate over time for conditions A, B, and C.

Figure 4.

Instantaneous corrosion rate variation of conditions A (ASTM D1141 at 25° – B = 8.46 mV), B (ASTM D1141 at 4°C – B = 12.94 mV) and C (Natural at 4°C – B = 11.80 mV) at 1bar and without agitation.

By analyzing the corrosion rate graphs obtained by means of the LPR technique and the values of the Stern-Geary constant, it is observed that, for conditions B and C, there was a tendency to reduce the rate, remaining below 0.050 mm/year, while for condition A they were above 0.075 mm/year. These data are fundamental to assess that the instantaneous corrosion rate of carbon steel in seawater tends to be lower at lower temperatures, such as 4°C. And this may be the result of the combination of thermodynamic and kinetic factors that affect the kinetics of electrochemical reactions, oxygen diffusion, and the mobility of aggressive ions such as chlorides (Cl⁻). In addition to the precipitation of corrosion products and possible limestone deposits, such as calcium carbonate (CaCO₃) and magnesium hydroxide (Mg(OH)₂), which can form protective barriers on the surface of carbon steel, as mentioned by Möller et al.² Initial, intermediate and final values of the corrosion rate of A36 steel, obtained by means of the LPR technique, were compiled. This information is presented in Figure 5, expressed in millimeters per year, for different immersion periods.

Figure 5.

Comparison of the instantaneous corrosion rate of conditions A, B, and C at different immersion times, obtained at open circuit potential (OCP).

It is important to point out that the corrosion rate estimated by LPR exhibited a trend to lower values at OCP, compared to values commonly reported in the literature. This behavior can probably be understood as the effect of corrosion products accumulated at the interface carbon steel/seawater, due to the stagnant condition of the environment adopted. These products are not adherent and cannot provide protection against corrosion. However, on a qualitative basis, the attenuation of corrosion promoted by lowering the temperature can be confirmed from the results.

Cathodic protection by potentiostatic control was used, applying two polarization potentials: −750 mV_Ag/AgCl, referred as alternative or attenuated potential and −810 mV_Ag/AgCl, value slightly below the limit prescribed by international standards such as DNV-RP-B401, ISO 15589-2 and NORSOK M-CR-503.^8,9,30 Figure 6 shows that the current densities from the applied protection potentials remained cathodic throughout the tests. This point is relevant, necessary to confirm the cathodic characteristics of the applied potentials.

Figure 6.

Cathodic current density as a function of time for the polarization potentials. (a) Variation of current density for the potential of −750 mV_Ag/AgCl; (b) Variation of current density for the potential of −810 mV_Ag/AgCl.

It can be observed that the current densities required at a lower temperature of 4°C were always below the values registered at 25°C. This result confirms the influence of temperature on the cathodic polarization demand when seawater is at lower temperature, corresponding to deeper water conditions.

The analysis of the variation of the cathodic current density with time reveals distinct patterns for the imposed potential. As shown in Figure 6(a), under the alternative potential of-750 mV_Ag/AgCl the current density required to maintain cathodic protection exhibits significant stability, remaining at average values of approximately −20 μA/cm² without the progressive reduction with time documented in the specialized literature.^8,9 Figure 6(b) presents the similar result obtained at −810 mV_Ag/AgCl. At 25°C, the current density remains relatively constant (on average −30 μA/cm²), while at 4°C a gradual reduction is observed over the test period. This phenomenon agrees with the mechanisms described by Möller et al., Nezgoda et al.^2,17 who attribute such behavior to the progressive formation of a protective layer composed by carbonates and hydroxides (CaCO₃ and Mg(OH)₂), whose precipitation is favored at lower temperatures.

Figure 7.

Corrosion rates (mm/year) of carbon steel in synthetic and natural seawater under different temperatures (25 °C and 4 °C), comparing open circuit potential (OCP) and two cathodic protection potentials: −750 mV_Ag/AgCl and −810 mV_Ag/AgCl. The data demonstrate that the alternative potential (−750 mV_Ag/AgCl) provides protection levels comparable to the normative potential (−810 mV_Ag/AgCl) in all tested conditions. *LD- Detection limit of the analytical balance - < 0.001 mm/year.

The results obtained partially corroborate the findings of Larché et al.,²⁹ who observed progressive reductions in current density for more negative cathodic potentials (−867 mV_Ag/AgCl). However, it is important to highlight that, as demonstrated by Liduino et al.,³¹ the formation and effectiveness of these protective layers critically depend not only on the potential applied, but also on the specific composition of the corrosive medium. Despite the theoretical relevance of these mechanisms, in the present study no in-depth investigations were conducted on the formation of these protective layers. The visual inspection carried out did not allow us to confirm the uniform presence of the limestone film under the experimental conditions analyzed.

Figure 7 presents the results of the mass loss tests. The results clearly demonstrate the efficiency of cathodic protection, with significant reductions in corrosion rates compared to the unprotected system (OCP). It is important to point out the higher corrosion rate values registered at OCP in comparison with results obtained by LPR. This difference can be attributed to the removal of the corrosion products carried out on mass loss tests, since the ML methodology includes an acid pickling step that eliminates these products, while in LPR they may remain on the surface and influence the electrochemical response. Therefore, the apparent discrepancy does not reflect an incorrect choice of the B parameter, but rather the intrinsic methodological differences between the two techniques. Data obtained with mass loss are more consistent and not so conservative as the estimation based on LPR measurements.

From the results, under condition A (25 °C, synthetic seawater), the OCP corrosion rate was 0.110 ± 0.009 mm/year, similar to values reported in the literature, such as those presented by Leeds and Möller et al.,^2,32 which reported corrosion rates above 0.100 mm/year under unprotected conditions. When applying the potentials of −750 mV_Ag/AgCl and −810 mV_Ag/AgCl, the corrosion rate is drastically reduced to 0.006 ± 0.005 mm/year and 0.004 ± 0.002 mm/year, respectively. The proximity between these values reinforces that the alternative potential of −750 mV_Ag/AgCl provides a protection level comparable to the standard potential of −810 mV_Ag/AgCl. Both corrosion rate values are considerable below the OCP values and confirm the efficiency of the cathodic polarization as an adequate technology capable of preserving carbon steel integrity in seawater. It is remarkable that the corrosion rate registered under the assumed immunity domain is not null, as expected if strictly equilibrium thermodynamics concepts could be applied.

This carbon steel behavior in seawater is also in compliance with findings reported by Larché et al.,²⁹ who showed that even potentials less negative than the normative criterion ( −667 mV_Ag/AgCl) can significantly reduce corrosion, from 0.170 to 0.060 mm/year. These results support the technical feasibility of adopting the −750 mV_Ag/AgCl alternative potential, as advocated by Dugstad et al. and Googan,^11,33 who emphasize the importance of flexibility in protection criteria depending on operational conditions.

In Condition B (4 °C, synthetic seawater), the corrosion rate under OCP was 0.035 ± 0.009 mm/year, and for both applied potentials, the values were below the mass loss detection limit (< LD), indicating effective cathodic protection. In Condition C (4 °C, natural seawater), the OCP value was 0.061 ± 0.004 mm/year, decreasing to 0.002 ± 0.003 mm/year and 0.002 ± 0.001 mm/year under −750 mV and −810 mV, respectively. Once again, the similarity between values confirms that −750 mV is sufficient to ensure effective cathodic protection under the evaluated environmental conditions.

The literature on carbon steel corrosion indicates that corrosion rates tend to increase with temperature, due to enhanced electrochemical kinetics and increased oxygen diffusion at moderate temperatures (between 25 °C and 30 °C), as discussed by Melchers.³⁴ This trend is confirmed in the present study, where Condition A (25 °C) showed the highest corrosion rate, while Conditions B and C (4 °C) presented significantly lower values. Although oxygen solubility increases at lower temperatures, the kinetics of the oxygen reduction reaction are considerably reduced, which may explain the lower corrosion rates observed, as discussed by Larché et al., Melchers and Rocha et al.^12–14 Notably, Rocha et al.¹³ also reported reduced corrosion rates between 4 °C and 10 °C even in the absence of cathodic protection, reinforcing the temperature effect observed in this study.

Furthermore, the results obtained are similar to those reported by Refait et al.,²⁵ who recorded corrosion rates between 0.006 and 0.008 mm/year in systems operating at more negative potentials (−950 to −1000 mV_Ag/AgCl). This similarity suggests that less negative potentials can also provide adequate protection depending on system conditions, as further confirmed by Refait et al.³⁵ in studies using synthetic seawater. These authors also emphasize that the formation of protective calcareous layers occurs mainly at more negative potentials, below −900 mV_Ag/AgCl. However, despite the mentioned positive effects of calco-magnesian deposits at more cathodic potentials, the drawbacks of an excessive overpotential are related to a more intense hydrogen evolution expected at the vicinity of the potential values assumed as overprotection by the standards. The consequence could be the increased risk of hydrogen embrittlement of structural carbon steels. Considering this, moving the cathodic polarization potential towards lower overpotentials can be considered as an alternative to reduce risks of adverse hydrogen effects.^8,25,36,37

To complement the experimental results, the next step was the assessment of corrosion morphology. The test samples were subjected to pickling and surface inspection after the experiments, including visual analysis, optical microscopy, and digital imaging using stereomicroscope and digital SmartZoom 5-ZEISS equipment. Corrosion of carbon steel in seawater typically develops uniformly, resulting in a homogeneous attack across the entire metal surface, in other words, leading to homogeneous thickness reduction. Figure 8 shows the surface samples after pickling, revealing a corrosion morphology consistent with uniform attack under open circuit potential (OCP), the most severe in terms of corrosion rate. Similar morphologies were observed on the surface of the samples subjected to cathodic protection. No signs of localized corrosion were observed, confirming the assumption made and the effectiveness of the cathodic protection imposed.

Figure 8.

Surface morphology of ASTM A36 carbon steel specimens after 30 days of electrochemical testing, showing corrosion features developed under each experimental condition. (a) Condition A - synthetic seawater at 25°C, pressure of 1 bar and without agitation. (b) Condition B - synthetic seawater at 4°C, pressure of 1 bar and without agitation. (c) Condition C – Natural seawater at 4°C, pressure of 1 bar and without agitation.

Conclusions

The results of the present study confirm the possibility of optimizing cathodic protection criteria for carbon steel equipment operating in subsea environments, demonstrating the feasibility of using alternative potentials traditionally recommended by international standards. Based on electrochemical measurements and corrosion rate assessments, it was concluded that ASTM A36 carbon steel immersed in seawater can be effectively protected at −750 mV_Ag/AgCl cathodic potential, less negative than the normative recommended threshold, which is −800 mV_Ag/AgCl.

Thermodynamic simulations and laboratory results revealed that parameters such as ferrous ion activity and temperature significantly influence the potential required to achieve the immunity domain. In particular, at 4 °C, the corrosion rate was virtually null in the protected samples; while at 25 °C a very low residual rate was recorded, ranging from 0.004 to 0.006 mm/year. These findings confirm the lower cathodic current demand and the slowed down corrosion kinetics at lower temperatures.

Visual inspections confirmed the electrochemical and mass loss results since uniform corrosion was identified in the surface of the samples. Furthermore, even in the absence of calcareous deposits formation, samples exposed to −750 and −810 mV_Ag/AgCl, the corrosion protection achieved was effectively maintained.

The results indicate that moderate cathodic polarization, −750 mV_Ag/AgCl, can ensure adequate protection for carbon steel structures. The results suggest the reassessment of the universal applicability of the −800 mV_Ag/AgCl criteria, leading to more flexible acceptance limits for CPS operation. The adoption of less conservative potentials — particularly in deep and cold seawater operations — can significantly reduce anode consumption, minimize subsea interventions, and consequently lower operational costs and environmental impact. The most important conclusion is that the protection level achieved was equivalent under standard and alternative cathodic polarization potentials. And both values are significantly lower than the CR obtained at OCP, without protection. The threshold value informed 10 µm/year is extremely conservative and probably not achievable under real seawater exposure conditions for carbon steels. Very low uniform corrosion rates can be easily compensated by the design criteria of carbon steel subsea equipment by using an extra wall thickness in accordance with the service life expected.

Footnotes

Acknowledgment

We gratefully acknowledge the support of the Federal University of Rio de Janeiro, COPPE/UFRJ – Instituto Alberto Luiz Coimbra de Pós-Graduação e Pesquisa de Engenharia, and the Brazilian funding agencies CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), and FAPERJ (Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro).

ORCID iDs

Evellyn de Oliveira Brazil

Eduardo A de Souza

Simone Louise DC Brasil

José AC Ponciano Gomes

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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