porosity, Corrosion behavior, Tafel polarization, NaCl solution, Pipeline steel, Electrochemical corrosion.

Alloyed low-carbon steels have become common materials across many sectors, such as automobiles, the oil industry, pipelines, and aerospace, because of their favorable mechanical behavior, weldability, and lower prices [1]. Among those low-carbon steels, one type, ST37, is defined by DIN 17100 and has become one of the most common alloys used in the oil industry [2].

High-Frequency Welding (HFW) or High-Frequency Electric Resistance Welding (HF-ERW) is a crucial manufacturing technique for producing longitudinal seam pipes from steel strip. This technique involves applying high-frequency alternating current (100-400 kHz) to generate localized resistive heating at the edges of cold-formed steel strips, then forging the strip under pressure without filler metal. Some of the strengths of HF-ERW include reduced heat input, precise dimensions, higher efficiency, and shorter delivery time, among others, as explained in detail by Brito et al. [3].

HFW welds require careful, accurate adjustment of welding parameters to ensure high-quality welds and the integrity of the welds. As noted by some researchers, a frequency of 150 kHz, compression force of 4 marks, and Vee angle of 5° would provide the best mechanical results when using HF-ERW on API X52 microalloy steel joints [4].

Some research studies have examined how welding current affects weld performance, especially in low-carbon steels. For instance, Jannifar et al. [2] determined the impact of welding current on the corrosion resistance and hardness of ST37 steel welded joints (at 75, 85, and 95 amperes). As the welding current increased, corrosion resistance decreased, making the weld zone more susceptible to corrosion. The maximum corrosion rate was 0.10516 mm/year in samples with a welding current of 95 amperes, whereas the highest hardness was observed in the weld metal at 75 amperes [2].

The uncontrolled use of welding parameters can lead to weld defects. Non-Metallic Inclusions (NMIs) were named as a reason for defects appearing in High-Frequency Resistance-Welded (HFRW) joints. Kazakov et al. [5] conducted detailed research on the chemical composition and origin of NMIs in defects of pipe joints fabricated by HFW technology. As a result, the following conclusion was made: The composition of NMIs greatly differs since it comes from indigenous deoxidation products of the spinel type of MgO. Al₂O₃ (8%), products of steel modification due to calcium addition (62%), residues of mould flux (4%), and from other exogenous sources. Besides, Kazakov et al. [5] reported that hook cracks prevail among various defects in High-Frequency Induction Welded (HFIW) pipes and that there is a direct relation between NMIs along the weld bondline and the appearance of the mentioned crack.

Radiographic testing is commonly used for non-destructive testing of longitudinal welded pipes. At the same time, Shleenkov et al. [6] discovered that non-metallic inclusions of liquation strips appearing on the edge and falling into the welding area could be found in X-ray inspection, but are not recognized as defects. The misinterpretation results from the fact that the grooves' location within the zones of the welded joint corresponds to the X-ray diffraction spectra. In contrast, no such defects were found with other control procedures and metallographic studies [6].

The weld metal's corrosion resistance is a critical factor that dictates the pipeline's performance. The welding process causes localized heating and solidification, which give the weld metal different properties, leading to its corrosion. Electric-resistance-welded pipes made of carbon and low-alloy steels enriched with sulfur exhibit grooving corrosion under neutral-salt-water conditions; this localized corrosion occurs at the weld zone [7]. The potential corrosion rate of grooving is evaluated using an accelerated potentiostatic test, with sulfur concentration being an important factor in assessing susceptibility to this type of corrosion [7]. On the other hand, HFI welded line pipes made of carbon and low-alloy steels resist groove corrosion due to their low sulfur content [7].

Additional research has also examined the effect of microstructure on toughness in HFI-welded pipes and analyzed different parameters, including microstructure, grain size, precipitates, hardness, inclusions, and texture, to correlate weld microstructure with toughness of the HFI-welded joint [8].

A case study by Tyrala and Pawlowski [9] focused on a premature corrosion failure of an HF induction seam-welded steel pipe used in a central heating system. Microscopic investigation by these authors showed that the failure was due to a problem associated with the HF induction welding process, namely, a so-called cast weld, a weld line containing a large amount of iron oxides due to poor ejection of molten metal at the bond line. Tyrala and Pawlowski [9] noted that the pipe failed by longitudinal cracking less than 2 years after operation due to a weld-line failure. It can thus be concluded that weld defects such as oxides trapped in the HF-ERW seam cause longitudinal cracking during service.

Pawłowski et al. [10], through further research on seam-welded galvanized steel cold-water pipes, revealed that premature failure due to corrosion occurs within 18-24 months of installation in service. In a separate study by Tyrała and Pawłowski [11], pitting corrosion was observed solely in regions of seam-welded L235 steel pipes with many microvoids, suggesting that high levels of trapped hydrogen in these pipes could lead to early failure. These findings reveal the importance of welding defects, especially oxide inclusions, in reducing the lifespan of steel pipes.

There is substantial evidence suggesting Post-Weld Heat Treatment (PWHT) as an efficient approach to improve weld properties. Some novel PWHTs involving normalization, quenching, and tempering procedures have been successfully applied to improve toughness in high-frequency electrical resistance-welded welds [12]. In the case of API X60 steels, quenching and tempering heat treatment (600ºC/30min) was successful in increasing absorbed energy twofold compared to commercial treatments, while maintaining the hardness and ultimate strength of the weld at 33J, compared to commercial heat treatments at -25ºC (17J) [12].

Due to the high degree of applicability of ST37 steel in various industries as well as the necessity for good weldability, it has become necessary to conduct a thorough study on how HFW parameters, especially current density and voltage, affect the creation of such defects as can be revealed using radiography and, subsequently, their strength and durability characteristics. The present research focuses on filling this knowledge gap by studying the aforementioned factors in the case of ST37 steel pipes manufactured using HFW.

For the current work, a 12-inch steel pipe with a 6 mm wall thickness, manufactured from ST37 low-carbon steel, was used (Fig. 1). The steel pipe was produced using the HFW technique via induction current transfer in a Siemens welding facility in Germany. The HFW process was performed at a constant frequency of 180 Hz; however, varying current and potential values were applied for the conditions specified in Table 1. The pipes were manufactured in accordance with API specifications. The chemical analysis of the ST37 steel sample was conducted using an Optical Emission Spectrometric technique, as shown in Table 2.

Radiographic images of the weld cross-sections were taken and analyzed using an industrial radiographic viewer system to determine porosity distribution and weld defects. Radiographic examination was performed to detect discontinuities in the metal, such as gas porosity and high-penetration defects, that were created during the HFW process.

Circular specimens with a surface area of 1 cm² were cut so that the weld line was at the center of each specimen for measuring corrosion rate using electrochemical corrosion measurement. The corrosion properties of the HFW specimens were tested using electrochemical Tafel polarization. Experiments were carried out on a standard three-electrode electrochemical cell immersed in 3.5% NaCl solution at 25 °C. A Saturated Calomel Electrode (SCE) was used as the reference electrode, graphite as the Auxiliary Electrode (AE), and an HFW specimen as the Working Electrode (WE).

Electrochemical tests were conducted on a state-of-the-art potentiostat/galvanostat instrument (EG&G PARSTAT 2273) that utilized the PowerSuite software from AMETEK. Prior to the polarization test, the samples were soaked in the electrolytic solution for 30 minutes to achieve a steady (OCP). Electrochemical Tafel polarization studies were performed over a potential range of -0.25 V to 0.5 V with respect to OCP at a scan rate of 1 mV s⁻¹.

The values of Icorr (corrosion current density), Ecorr (corrosion potential), βα (anodic Tafel slope), and βc (cathodic Tafel slope) were determined from the polarization curves through the use of the PowerSuite electrochemical analysis software. The corrosion resistance properties were assessed using the Stern–Geary equation, which relates polarization resistance and corrosion current density as follows.:

1 βα. βc

icorr = [ ] [ ], (1)

2.303 Rp βα + βc where icorr is the corrosion current density (A/cm2), Rp is the polarization resistance (Ω cm2), and βα, βc represent the anodic and cathodic Tafel slopes, respectively.

The corrosion current density was determined by extrapolating the linear Tafel regions of both anodic and cathodic branches to the corrosion potential. In the polarization curves, the anodic and cathodic regions exhibited linear behavior at approximately ±50 mV relative to the open circuit potential, corresponding to the Tafel regions.

In this work, the effects of welding current and voltage on the radiographic properties and corrosion resistance of HF-welded ST37 steel pipes have been investigated. Radiographic examination of the weld has shown that increasing the welding current and voltage resulted in a noticeable increase in dispersed porosity in the weld zone. The formation of these defects was due to high heat input during the welding process. Increased heat input increased hydrogen solubility in the melted weld and led to the formation of pores during cooling.

To assess the electrochemical corrosion resistance of the specimens, Tafel polarization curves in 3.5 wt.% NaCl solution has been constructed. According to the results, increased porosity in the weld zones was accompanied by elevated corrosion current density and low polarization resistance. On the other hand, the sample welded at a lower current showed better corrosion resistance than the sample welded at a higher current. This implies that high current intensity during welding is detrimental to weld quality and corrosion resistance in HF-ERW ST37 steel pipes. All in all, the results of this study clearly indicate that optimizing HFW process parameters is critical for preventing weld defects and improving the corrosion resistance of the pipe used in corrosive conditions.

The authors declare that they have no competing interests.

The data supporting the findings of this study are available within the article.

This research did not receive any external funding.