Metal Release and Corrosion Resistance of Different Stainless Steel

CORROSION SCIENCE SECTION

Metal Release and Corrosion Resistance of Different Stainless Steel Grades in Simulated Food Contact N. Mazinanian,‡,* G. Herting,* I. Odnevall Wallinder,* and Y. Hedberg‡,*

ABSTRACT A new technical guideline has been implemented by the Council of Europe (CoE) to ensure the stability and safety of food contact articles of metals and alloys, using 5 g/L citric acid (pH 2.4) and artificial tap water DIN 10531 (pH 7.5) as food simulants. The objectives of this study were: (i) to quantify the extent of metal release from austenitic (grades AISI 201, 204, 304, and 316L), ferritic (grades AISI 430 and EN 1.4003), and lean duplex stainless steel (grade EN 1.4162) in citric acid (5 g/L, pH 2.4) and in artificial tap water (pH 7.5); (ii) to compare the release of metals to the surface oxide composition, the open circuit potential–time dependence, and the corrosion resistance; and (iii) to elucidate the combined effect of high chloride concentrations (0.5 M NaCl) and citric acid at pH 2.2 and 5.5 on the extent of metal release from AISI 304 with and without prior surface passivation by citric acid. Exposures of all stainless steel grades in citric acid and artificial tap water up to 10 d (at 70°C/40°C) resulted in lower metal release levels than the specific release limits stipulated within the CoE protocol. For all grades, metals were released at levels close to the detection limits when exposed to artificial tap water, and higher release levels were observed when exposed to citric acid. Increased surface passivation, which resulted in reduced metal release rates with time, took place in citric acid for all grades and test conditions (e.g., repeated exposure at 100°C). There was no active corrosion in citric acid at pH 2.4. Fe (in citric acid) and Mn (in all solutions, but mostly tap water) were

‡

*

Submitted for publication: February 17, 2016. Revised and accepted: March 16, 2016. Preprint available online: March 17, 2016, http:// dx.doi.org/10.5006/2057. Corresponding author. E-mail: [email protected] (Y. Hedberg), [email protected] (N. Mazinanian). KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas väg 51, SE-10044 Stockholm, Sweden.

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preferentially released, as compared to their bulk alloy content, from all stainless steel grades. Ni was released to the lowest extent. 0.5 M NaCl induced a very low (close to detection limits) metal release from grade AISI 304 at pH 5.5. When combined with citric acid (5 g/L) and at lower pH (2.2), 0.5 M NaCl induced slightly higher metal release compared to citric acid (pH 2.4) alone for coupons that were not prepassivated. Pre-passivation in 5 g/L citric acid (pH 2.4) at 70°C for 2 h largely reduced this solution dependence. Prepassivation resulted in an up to 27-fold reduced extent of metal release in solutions containing citric acid and/or NaCl at pH 2.2 to 5.5, and resulted in improved reproducibility among replicate samples. KEY WORDS: chloride, citric acid, corrosion resistance, food, food safety, metal release, stainless steel, surface oxide

INTRODUCTION Stainless steel is an iron (Fe) based alloy with at least 11 wt% chromium (Cr),1 which may also contain several other alloying elements, such as nickel (Ni), molybdenum (Mo), and manganese (Mn).2 Stainless steel is widely used in food- and beverage-relevant applications because of its high corrosion resistance in combination with good mechanical properties.3 The corrosion resistance of stainless steels is a result of the existence of a very thin self-healing chromium-rich passive surface oxide (sometimes called passive film or passive layer), typically 1 nm to 3 nm thick.4-5 In this paper, the passive film is denoted the surface oxide and contains usually divalent or trivalent Fe and trivalent Cr oxides, hydroxides, and/or oxyhydroxides.4,6-7 Nickel is enriched in its metallic form beneath the

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surface oxide, but usually is not present in the surface oxide.4,8-11 Oxides of manganese and molybdenum can be present in the surface oxide of some stainless steel grades and at some environmental conditions.4-5,7-8,12-13 The surface oxide composition, thickness, and other properties dynamically change with time and gradually adjust to the environment.4-6,11,14 For example, chromium is enriched in the surface oxide at acidic conditions.11,14-18 Passivation of the surface oxide in citric acid has, with significantly reduced released metals as a consequence, been reported.13-14,19-20 The extent of released amounts of the main alloying constituents, Cr, Ni, Mn, and Fe, from stainless steel cookware depends on several factors including grade, cooking time, pre-use, and temperature.19,21 Different metal release mechanisms for the stainless steel surface in solutions of relevance for food applications, including electrochemical (metal corrosion/ oxidation), chemical/electrochemical (dissolution of the surface oxide), or physical processes (removal of metal or oxide particles via, e.g., friction), have recently been reviewed.19 In Europe, a new test guideline has recently been published by the Council of Europe (CoE) to ensure safety of metals and alloys in food contact.22 Main changes compared to the earlier available test, stipulated within the Italian Decree,23 are the use of citric acid (5 g/L, pH 2.4) instead of acetic acid (31.5 g/L, pH 2.4) to simulate contact with acidic foods. There is also greater freedom in the test setup to enable more application-realistic investigations. Specific release limit (SRL) values, based on available toxicological, daily intake, and/or sensitization data, have been stipulated in the guideline for metals of concern. These values are used in compliance tests for comparison with corresponding levels released from metals and alloys into the test medium at a given surface area to solution volume. Recent findings show that the CoE protocol test conditions provide similar, or even more aggressive, test conditions from a metal release perspective compared with the setup described by the Italian Decree.13 The main objectives of this study were: (i) to quantitatively assess the extent of released metals from austenitic (grades AISI 204 [UNS S20431],(1) 304 [UNS S30400], and 316L [UNS S31603]), ferritic (grades AISI 430 [UNS S43000] and EN 1.4003 [UNS S40977]), and lean duplex stainless steels (grade EN 1.4162 [LDX 2101†, UNS S32101]) exposed in citric acid (pH 2.4) and artificial tap water (pH 7.5),

(1)

†

UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International. Trade name.

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when following the CoE protocol and upon repeated exposure; (ii) to relate the extent of metal release to differences in surface oxide composition, the open circuit potential (OCP)–time dependence, and corrosion resistance of the same grades; and (iii) to investigate the combined effect of high chloride concentrations (0.5 M NaCl) and citric acid at pH 2.2 and 5.5 on the extent of metal release with and without prior surface passivation by citric acid (elucidated for AISI 304). The pH 2.2 solution simulates very harsh conditions.

MATERIALS AND METHODS Materials Six different stainless steel grades of 2B surface finish (bright cold-rolled, annealed, pickled, and skinpassed sheet) and sheet thicknesses ranging from 1 mm to 2.5 mm were supplied by the International Stainless Steel Forum. Another stainless steel grade, AISI 201 (UNS S20100), with a different surface finish, 2D (dull cold-rolled, annealed, and pickled), is included for comparison with an earlier study.13 The microstructure and nominal bulk composition of the different grades are presented in Table 1. Additional information on the duplex microstructure of the grade EN 1.4162 is available in the Appendix (Figure A1).

Metal Release Studies and Experimental Conditions All coupons were prepared with a total geometric surface area of approximately 6 cm2 (each defined separately). As-received surfaces (2B or 2D) were investigated following the CoE protocol.22 All cutting edges of the as-received coupons were abraded using 1200 grit SiC paper. The coupons were then cleaned ultrasonically in ethanol and acetone for 5 min each, subsequently dried with cold nitrogen gas, and aged (stored) for 24±1 h in a desiccator (at room temperature). As the thickness of the coupons varied between 1 mm and 2.5 mm, the edge area to total surface area ratio varied between 10% (grades AISI 201, 204, and 304), 15% (grade EN 1.4003), 19% (grade AISI 430 and EN 1.4162), and 24% (grade 316L). During exposure in solution, the surface area to solution volume ratio was kept constant at 1 cm2/mL. Triplicate coupons and one blank sample (test solution only) were exposed in parallel for each grade, time period, and test solution. All exposures were conducted in an oven at stipulated temperatures (Torrsterilisator†, Termaks). All vessels were acid-cleaned in 10% HNO3 for at least 24 h, rinsed four times in ultrapure water (>18 MΩ·cm, Millipore†), and dried in ambient laboratory air. All chemicals used were of analytical grade (p.a.) or puriss p.a. grade (in the case of nitric acid, used to

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TABLE 1 Microstructure and Nominal Bulk Alloy Composition of As-Received Sheets of Different Investigated Stainless Steel Grades Based on Supplier Information (wt%)(A) Name EN 1.4003 AISI 430 AISI 204 AISI 201 AISI 316L AISI 304 EN 1.4162 (A)

UNS

EN

Microstructure

Finish

Fe

Cr

Mn

Ni

Cu

Mo

N

C

S

S40977 S43000 S20431 (+Cu) S20100 S31603 S30400 S32101

1.4003 1.4016 1.4597 (+Cu) 1.4372 1.4404 1.4301 1.4162

Ferritic Ferritic Austenitic Austenitic Austenitic Austenitic Duplex

2B 2B 2B 2D 2B 2B 2B

Bal. Bal. Bal. Bal. Bal. Bal. Bal.

11.2 16.0 15.9 16.9 16.9 17.9 21.4

1.0 0.3 9.1 5.8 1.3 1.2 4.8

0.4 0.1 1.1 3.6 10.1 9.0 1.6

0.05 0.04 1.6 0.4 0.5 0.4 0.3

0.01 0.02 0.15 0.21 2.0 0.36 0.28

0.05 0.03 0.19 0.15 0.05 0.04 0.22

0.02 0.03 0.09 0.11 0.02 0.04 0.02

0.0007 0.0016 0.0040 0.0020 0.0006 0.0029 0.0010

Bal.: balance, AISI: American Iron and Steel Institute, UNS: Unified Numbering System, EN: European Standard.

TABLE 2 Food Simulants Stipulated in the CoE Protocol22 Type of Food Aqueous, alcoholic, or fatty food Acidic foods (pH ≤ 4.5) (A)

Simulant Artificial tap water DIN 10531(A) (pH 7.5) 5 g/L citric acid (pH 2.4)

DIN 10531:40 It contains 0.12 g/L NaHCO3, 0.07 g/L MgSO4·7H2O, 0.12 g/L CaCl2·2H2O.

acidify solution samples to a pH of