Effects of phosphoric acid based rust converter on stainless steels for anti-corrosion applications

Jeong Seok Park Interdisciplinary School of Green Energy Ulsan Graduate School National Institute of Science and Technology The performance of phosphoric acid-based rust converter was investigated on type 304 stainless steel and duplex stainless steel 2205 in terms of corrosion inhibition and the effects of removing the scale. In order to make a comparative study, the inhibition efficiency of the as-received and pre-oxidized steels in the presence and absence of the rust converter was measured. From the electrochemical studies, the inhibitory effect of the rust converter generally depends on the condition of the specimens.

Rust converter applied to as-received steel was observed to play a role as anodic type inhibitor.

Introduction

Background

Objectives

Xueming Li, 2005, 'Synergistic inhibition between OP and NaCl on the corrosion of cold-rolled steel in phosphoric acid', Materials Chemistry and Physics, vol. Libin Tang, 2006, 'Synergistic effect between 4-(2-pyridylazo)-resorcinol and chloride ion on the corrosion of cold-rolled steel in 1.0 M phosphoric acid', Applied Surface Science, vol. Collazo, 2010, 'The corrosion protection mechanism of rust converter: an electrochemical impedance spectroscopy study', Electrochimica Acta, vol.

Nasrazadani, 1997, 'Application of infrared spectroscopy to a study of the interactions of phosphoric and tannic acids with magnetite (Fe3O4), goethite (α-FeOOH) and lepidocrocite (γ-FeOOH)', Corrosion Science, vol.

Fig. 1.1 Schematic view of (a) PWR power plant and (b) recirculating-type steam generator [20]

Literature review-

General characteristics of stainless steels

The second region observed in fig. 2.8, corresponds to the passive state of the metal. If it is assumed that the acoustic velocity in the foreign layer, and its density, are identical to those of quartz (cf. the assumption for the metal electrodes), a change in the thickness of the foreign layer is equivalent to a change in the thickness of the quartz crystal . Surface morphology of the phosphate conversion layers formed on pre-oxidized steels is represented in Fig.

4.21-28 show XPS spectra of major elements for as-received and pre-oxidized steels with and without rust converter. Lakatos-Varsanyi, 1998, "Effect of Phosphate on Repassivation of 304 Stainless Steel in Neutral Chloride Solution", Electrochimica Acta, vol. The capacitance of the layer corresponds to the stability of the passive layer formed after receiving the steels.

Table 2.1 Production and consumption of stainless steels in some of the major economics (production/consumption) [4]

Major problems of stainless steels

Mechanism of corrosion inhibition

Acid pickling of austenitic stainless steels

Corrosion tests

This resonant frequency is sensitive to mass changes (and other factors) of the crystal and its electrodes. Therefore, an alternating potential across the crystal causes vibrational motion of the quartz crystal with the vibrational amplitude parallel to the crystal surface and in the x direction. It is important to note that the direction of the crystal vibration is critical for liquid phase applications.

The acoustic wavelength is longer in the composite resonator due to the greater thickness, resulting in a lower frequency compared to the quartz crystal [63]. The thickness of the oxide layer was estimated by argon ion sputtering in 4 second intervals. The percentage inhibition efficiency (% IE) of the rust converter was calculated according to the following equation.

However, it is seen from Table 5.2 that rust converter had the effect of lowering the anodic current indicated in the larger values of the anodic Table Slope (βa) compared to pre-oxidized steel without rust converter. This weight gain showed that the iron ions led to the formation of the protective layer [9]. EPMA was performed to analyze the composition shown in Table 5.4 of the phosphate conversion layer.

Therefore, XPS was also performed to provide information on the nature and chemical composition of the conversion layer on as-received and pre-oxidized steels with or without a rust converter. In the as-received steels, the dominant peaks were responsible for the metallic and trivalent states of chromium. H3PO4 + FeOOH → FePO4 + 2 H2O (3) In the case of pre-oxidized steels, it is concluded that the corrosion inhibition of stainless steels is mainly enhanced by the formation of an iron complex.

The influence of rust converter based phosphoric acid with pH 0.4 on stainless steel as received leads to the improvement of the main passive layer in the formation of a mixture of Cr2O3, Cr(OH)3 and CrPO4.

Rationale and Approach

Problem definition

Among the surface cleaning techniques, abrasive blasting provides the best level of oxide removal and leaves the surface roughened suitable for subsequent surface treatment to improve corrosion resistance. However, abrasive blasting cannot always be applied due to the location of the equipment to be treated and its geometry. The main function of a rust converter is to react with iron oxides that cannot be completely removed from the surface, leading to a layer where coating systems can be applied.

Among the rust converters, special attention can be paid to those based on tannic and phosphoric acid, which are environmentally safe, in contrast to toxic inhibitors such as red lead or zinc chromates. The research on the protection efficiency of rust converters is controversial depending on the condition, which may explain the different nature of the iron oxide layer to be converted. Although there are certain agreements that the rust converter reacts with iron oxides, leading to phosphate conversion layer and high concentration of phosphoric acid solution leads to higher protection efficiency of the rust converter, many studies are contributed to carbon steel and low alloy steel not stainless steel Austenitic and duplex stainless steel are widely used in various industries as structural materials due to the excellent mechanical properties and high corrosion resistance.

Despite their excellent mechanical properties and high corrosion resistance, these stainless steels suffer from atmospheric corrosion caused by marine environment and variation in humidity and elevated high temperature.

Goal & Approach

Experimental method & materials

Surface analysis of oxide layers

XRD and XPS depth profile

Analysis of oxide conversion layer

Electrochemical test: Polarization, EIS and EQCM
EPMA & XPS analysis

In order to form the conversion layer, pre-oxidized steel was immersed in 50ml of the commercial rust converter (RUST CLEAN-N®) based on phosphoric acid with pH 0.4, according to the technical information provided by the producer. Before electrochemical testing, all the samples were cleaned with distilled water before being placed in the corrosion cell. Impedance measurements were made using a Solartron SI 1260 HF frequency response analyzer along with the EG&G 273A potentiostat-galvanostat.

All impedance measurements were performed at open circuit potential (OCP) after immersing the test samples in the test electrolyte for 60 minutes. The EQCM method for monitoring mass changes is based on the reverse piezoelectric effect in which a voltage is applied to an SS 304 covered with quartz crystal, causing physical deformation. For AT-cut quartz crystal vibrating in a thickness shear mode, the equation follows:

D = - D Where f0 is base resonance frequency of the crystal, A is Area, μ is shear modulus of quartz and ρ is density of quartz. In this study, AT-cut quartz crystal coated with a deposit of SS 304 supplied by SEIKO EG & G of Japan was used to three-electrode cylinder-type corrosion cell. Counter electrode was a graphite rods, the saturated calomel electrode was used as the reference electrode.

Experiments were performed in three steps, 1) Open Circuit Potential (OCP), 2) Bias (from -0.5 to 1.8 VSCE), 3) Open Circuit Potential, using an EG&G 273A potentiostat-galvanostat i coupled with SEIKO EG & G QCM922 for frequency acquisition. Buttry, 1992, 'Measurement of electrode surface interface with quartz crystal electrochemical microbalance', American Chemical Society, vol.

Fig. 4.1 Optical microstructure of stainless steel 304 (Etchant: Acetic glyceregia)

Results

Surface analysis of oxide layer

SEM morphology
XRD analysis & XPS depth profile

Analysis of oxide conversion layer

Potentiodynamic polarization, EIS and EQCM
EPMA analysis
XPS analysis

The presence of rust converter with ash receiving steel causes decrease in the corrosion rate. Anodic and cathodic polarization curves for pre-oxidized steel were obtained with and without rust converter. Compared to as-received steel, polarization curves of pre-oxidized steel do not show the passive region.

The XPS peaks for the Cr 2p energy level recorded in the binding energy range 574–580 eV for the as-received and preoxidized steels with and without rust converters are shown through Figs. Deconvolution of the Cr 2p energy level gave three main peaks for the as-received steels and two peaks for the preoxidized steels. 5.23-26 show the Cr 2p peaks for preoxidized steels with and without rust converters.

In preoxidized steels with rust converter, signal of Cr3+ corresponding to chromium hydroxide Cr(OH)3, chromium phosphate CrPO4 is stronger than chromium oxide (Cr2O3) in preoxidized steels without rust converter. 5.21-28 were recorded for as-received and preoxidized steel with and without rust converter in the binding energy range 528-547 eV. Deconvolution of O 1p energy level gave two main peaks for as-received and preoxidized steel and three peaks for as-received and preoxidized steel with rust converter.

In the as-received and pre-oxidized steels with rust converters, the chemical states of oxygen were identified by three main elements corresponding to OH-, O2- and H2O. The P 2p points recorded for the as-received and pre-oxidized steels with and without rust converters in the binding energy range 132-136 eV are shown in Fig. 5.21-28. Phosphorus 2p peaks show a singlet at 133.8eV corresponding to 2p3/2 of phosphate in the as-received and pre-oxidized steels with rust converter.

While XPS peaks showed no P 2p peaks in as-received and pre-oxidized steels without rust converter [14].

Table 5.1 Electrochemical parameters of as-received SS 304 and DSS 2205

Discussion

In addition, the results of electrochemical quartz crystal microbalance (EQCM) in steels after receiving with a rust converter can also be considered as adsorption of phosphate ions on the surface. On the other hand, in the pre-oxidized steels with a rust converter, the iron compounds were the dominant peak. A schematic representation of the mechanisms for inhibiting the corrosion of the rust converter on the as-received and pre-oxidized steels is shown in fig.

Corrosion inhibition of phosphoric acid based rust converter on SS 304 and DSS 2205 in as-received and pre-oxidized condition was investigated. This product shows relatively high stability compared to those of stainless steels without rust converter. From surface analyses, including EPMA and XPS, the rust converter mainly reacts on the outermost Fe2O3 and changes the iron oxide protective layer.

Rust converter changes the protective iron oxide layer into iron oxyhydroxides (FeOOH) and iron phosphates (FePO4). However, conversion layer obtained by reaction between the iron oxides and the rust converter was superficial and did not improve the corrosion protection properties. From the potentiodynamic polarization tests, pre-oxidized steel without rust converter (Table 5.2) has better corrosion resistance than that of as-received steel (Table 5.1).

The rust converter appears to be a cathodic-type rust inhibitor due to the reduction in corrosion potential (Ecorr). Nevertheless, the rust converter has the effect of lowering the anodic current, which is manifested in higher values of βa compared to those of pre-oxidized stainless steels without the rust converter.

Fig. 6.1 Schematic representation for corrosion inhibition mechanisms of rust converter on as- as-received and pre-oxidized steels

Summary & conclusion