Why is it important to provide metal release data of alloy constituents

Metal Release from Stainless Steels and the

Pure Metals in Different Media

Gunilla Herting

Licentiate Thesis

Department of Materials Science and Engineering Division of Corrosion Science Royal Institute of Technology SE-100 44 Stockholm, Sweden

Stockholm 2004

Reviewer: Lena Wegrelius, Ph.D, Outokumpu Stainless AB, Avesta, Sweden

ISRN KTH/MSE—0468—SE+CORR/AVH. ISBN 91-7283-911-2

Abstract This study has been triggered by the fact that stainless steel is being increasingly used in new

applications, where possible environmental effects may be a matter of concern. When

stainless steel is exposed to a given environment, a key issue is the release of small amounts

of the main alloying elements iron, chromium, nickel and molybdenum. Published release rate

data of these elements turned out to be sparse. Furthermore, only little was known about the

role of different parameters that may affect the release rate, such as degree of alloying,

exposure time and surface finish. Hence, the aim of this study was to develop methodological

means and to provide accurate metal release rates of alloying constituents from different

grades of stainless steels- austenitic, ferritic and duplex- when exposed to selected

environments: artificial rain and synthetic body fluids. The results and discussion have been

summarised in this thesis by formulating and answering ten questions, all believed to be

crucial for the understanding of possible environmental effects of stainless steels.

Some common conclusions could be drawn, independent of stainless steel grade and exposure

condition. Iron was always preferentially released, and the release rates of chromium, nickel

and molybdenum (when measured) were significantly lower than of iron, also when

considering the bulk proportion of these elements. The release rate of all elements was

initially high and decreased with exposure time, mainly because of an observed enrichment of

chromium in the passive film formed.

The release rates of iron (2 µgcm-2week-1) and nickel (0.08 µgcm-2week-1) from stainless steel

from grades 304 and 316 exposed to artificial rain were much lower than corresponding rates

for the pure metals (750 µgcm-2week-1 released Fe and 15 µgcm-2week-1 released Ni), whereas

chromium exhibited similar release rates from stainless steel and the pure metal (0.1 µgcm-

2week-1). This implies that the common procedure to calculate release rates, based on the pure

metals and the nominal steel composition, significantly overestimates release rates of iron and

nickel from stainless steel, but not of chromium.

Total release rates from seven stainless steel grades in synthetic body fluid were found to

decrease with increasing alloy content in the following release rate order: grade 409 >> grade

430 > grades 316L ≈ 201 ≈ 2205 ≈ 304 > grade 310. The release rate was highly sensitive to

pH of the synthetic body fluid but only slightly sensitive to stainless steel surface finish.

Keywords: stainless steel, iron, chromium, nickel, metal release, artificial rain, synthetic body fluid

Preface

The following papers are included in this thesis:

I “A comparison of release rates of Cr, Ni and Fe from stainless steel alloys and the

pure metals exposed to simulated rain events”

G. Herting, I. Odnevall Wallinder and C. Leygraf

Journal of Electrochemical Society, 152 (2005).

II “Metal release from various grades of stainless steel exposed to synthetic body fluids”


Submitted to Journal of Environmental Monitoring (Aug. 2004).

III “Factors that influence the release of metals from stainless steels exposed to

physiological media”


Submitted to Corrosion Science (Oct. 2004).

In addition the following reports and papers have been completed but are not included in the

thesis.

“Release of chromium, nickel and iron from stainless steel exposed under atmospheric

conditions and the environmental interaction of these metals”,

I. Odnevall Wallinder, S. Bertling, G. Herting, C. Leygraf, Heijerick, D. Berggren and

P. Koundakjian, report 72 pages, www.eurofer.org (2004).

“Release rates of chromium, nickel and iron from pure samples of the metals and 304

and 316 stainless steel induced by atmospheric corrosion – a combined field and

laboratory study”,

I. Odnevall Wallinder, S. Bertling, G. Herting and C. Leygraf,

UN/ECE, workshop, Munich, Germany, May (2003).

“Release of Cr, Ni and Fe from stainless steel alloys and the pure metals”,

G. Herting, I. Odnevall Wallinder and C. Leygraf, Proc.

13th Scandinavian Corrosion Congress, Reykjavik, Iceland, April (2004).

http://www.eurofer.org/

Contents

1. Introduction……………………………...………………..……….... 1.1 Stainless steel – general information……………………………...…..…

1.2 Atmospheric corrosion of stainless steels..……………….………………

1.3 Metal release of alloy constituents from stainless steel during

atmospheric exposure………………...…….…...….……………..……....

1.4 Corrosion of stainless steel in synthetic body fluids……………....…..…

1.5 Metal release of alloy constituents from stainless steel in synthetic body

fluids……………………….....……………………….………………..….

1.6 Metal release of alloy constituents from stainless steel in different

media……………………………………………………….………….…..

2. Experimental…………...…..……..…………..……………………... 2.1 Investigated materials………..…………………….………………..……

2.2 Experimental set up…………………………………………….….……...

2.2.1 Metal release investigations in artificial rain (Paper I).…………...

2.2.2 Metal release investigations in synthetic body fluids

(Paper II+III)…………………………………………………….……….

2.3 Analysis of total metal concentrations................................…….………...

2.3.1 Inductively coupled plasma-mass spectrometry (ICP/MS)..……….

2.3.2 Graphite furnace atomic absorption spectroscopy (GF-AAS).….…

2.4 Analysis of surface film composition and surface roughness.....…….….

2.4.1 X-ray photoelectron spectroscopy (XPS)……………………….…….

2.4.2 Profilometry…………………………………………….………...………

2.4.3 Electrochemical impedance spectroscopy (EIS)………………….….

3. Summary of results………………………………………..…………3.1 Why is it important to provide metal release data of alloy constituents

from stainless steels?………………………..………………...…………..

3.2 How does the degree of alloying influence the metal release rate?……..

3.3 Is the metal release process of alloy constituents time-dependent?…..…

3.4 Is the release rate the same for all alloy constituents?.........………….....

3.5 Are laboratory investigations relevant for real exposure scenarios?……

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3.6 Are metal release rates of alloy constituents from stainless steels equal

to metal release rates from pure metals?………………….……………...

3.7 Can metal release rates from stainless steels be estimated based on the

nominal alloy composition and metal release data from pure metals…...

3.8 What kind of difficulties can be encountered during in-vitro

exposures in synthetic body fluids?…………………...………………….

3.9 Does the surface finish of stainless steels have any effect on metal

release rate?.........................................................……….……………..….

3.10 How can we use the generated metal release rates?..............…….……...

4. Main conclusions.......................................................………...............

5. Acknowledgement…………………………………………....………

6. References………………...……………………….….………………

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1. Introduction

Stainless steel is, due to its corrosion resistance and strength, one of the most widely used

materials today. Its area of application range from external constructions to implants and

increases continuously. However, the awareness in society of the potential adverse effect that

the release of metals from stainless may have on man and the environment has increased

during the last decades, and, hence, the need for accurate release data of alloy constituents

from stainless steel of different grades and environmental conditions. Such data and improved

understanding of the difference between alloys and pure metals are crucial for sustainable

decisions and legislative actions towards the use of metals.

This licentiate thesis provides quantitative data on metal release rates of alloy constituents

from stainless steel of different grades and surface finish when exposed in different media

ranging from artificial rain to synthetic body fluids. Relevant data on metal release in these

fluids is of importance for a correct evaluation or their potential influence on their

surrounding environment. The thesis also includes detailed investigations to compare metal

release rates from stainless steel alloys and corresponding metal release rates of each alloying

element exposed separately as a pure metal. Potential environmental and/or toxic effects of

metal release are not discussed within the context of this thesis; only total metal release rates

are presented without any consideration towards the metal chemical speciation.

The following 10 questions have been formulated during the course of this licentiate work, all

believed to improve the current knowledge on the metal release process of alloy constituents

from stainless steels in different media and possible environmental consequences.

1. Why is it important to provide metal release data of alloy constituents from stainless

steels?

2. How does the degree of alloying influence the metal release rate?

3. Is the metal release process of alloy constituents time-dependent?

4. Is the release rate the same for all alloy constituents?

5. Are laboratory investigations relevant for real exposure scenarios?

6. Are metal release rates of alloy constituents from stainless steels equal to metal

release rates from the pure metals?

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7. Can metal release rates from stainless steels be estimated based on the nominal alloy

composition and metal release data from pure metals?

8. What kind of difficulties can be encountered during in-vitro exposures in synthetic

body fluids?

9. Does the surface finish of stainless steels have any effect on metal release rate?

10. How can we use the generated metal release rates?

1.1. Stainless steel – general information

The term “stainless steel” relates to iron-based alloys of high corrosion resistance in

environments where both iron and low-alloyed steels would rust. To be called "stainless", the

steel must contain more than 10.5 wt% chromium [1]. However, for sufficient corrosion

protection, a minimum of 13 wt% chromium is desirable. The corrosion resistance increases

with increasing chromium content and is intimately related to the formation of a chromium-

rich surface film, usually referred to as the passive film, see below. Alloying elements, such

as nickel (promotes repassivation) and molybdenum (stabilises the passive film in chloride-

rich environments), have an influence of the passive film formation and, hence, on the

corrosion resistance.

Depending on bulk alloy composition and metallurgical phases of the microstructure, stainless

steels are divided into four main groups: austenitic, ferritic, duplex (austenitic-ferritic) and

martensitic stainless steels. Each group contains a number of different grades of stainless

steels. Around 200 grades are in commercial production. Austenitic grades contain typically

12-25 wt% chromium, 8-25 wt% nickel, and depending on grade, 0-6 wt% molybdenum [2,

3]. Austenitic steels, mainly grades 304 and 316, are the most widely used stainless steel

grades in applications that require a relatively high corrosion resistance, e.g. in external

applications and for pharmaceutical equipment.

Ferritic stainless steels are mainly alloyed with chromium, typically 13-30 wt-% [2, 3], and do

not contain any nickel. This group of steels are used in applications with lower demands on

corrosion resistance, such as interior constructions and containers.

Duplex stainless steels are typically composed of 18-25 wt% chromium, 4-7 wt% nickel and

up to 4 wt% molybdenum [2, 3]. Stainless steel grades of this group have generally high

corrosion resistance and high strength as a result of the combination austenitic and ferritic

microstructure. This group of stainless steels has also an improved resistance towards stress

corrosion cracking compared to plain austenitic and/or ferritic grades.

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Martensitic stainless steels are characterised by a higher carbon content and a chromium

content of 13-17 wt-% [2, 3]. These grades can be hardened and are hence commonly used for

e.g. knife blades. A compilation of the different groups of stainless steels and examples of

applications are presented in Fig.. 1.

Stainless steel

Austenitic Ferritic Duplex Martensitic

•External construction•Vehicles•Kitchen sinks•Chemical equipment•Food industry•Dinner ware•Implants•Jewellery

•Automobile exhaust pipes•Heat exchangers•Burners

•Knife blades•Bolts, nuts•Pump shafts•Turbine blades•Scissors

•On/off shoreapplications

•Brewery and piping systems

•Pulp and paper industry

Stainless steel

Austenitic Ferritic Duplex Martensitic

•External construction•Vehicles•Kitchen sinks•Chemical equipment•Food industry•Dinner ware•Implants•Jewellery

•Automobile exhaust pipes•Heat exchangers•Burners

•Knife blades•Bolts, nuts•Pump shafts•Turbine blades•Scissors

•On/off shoreapplications

•Brewery and piping systems

•Pulp and paper industry

Figure 1 Different groups of stainless steels and examples of areas of applications.

The passive film formed on the surface of stainless steel provides an effective barrier for

corrosion under normal conditions. However, the film is susceptible to local corrosion attacks.

The passive film, a few nm thick, forms spontaneously in contact with air and consists of

complex composition of iron oxides, chromium oxides (inner barrier layer) and chromium

hydroxides (outer layer) as main components [4, 5]. This film formation is the result of

hydrated complexes that are rapidly adsorbed on the surface followed by the formation of a

hydroxide film in the presence of water and, finally, the formation of an insoluble surface film

due to deprotonation of the hydroxide film [4]. If damaged, the passive film immediately

repairs itself under the influence of oxygen from water and/or air, thereby keeping the

material protected. A surface alloy layer enriched in nickel and molybdenum (e.g. grade 316)

is present adjacent to the passive surface film [4]. The thickness of the alloy surface layer is in

the same order of magnitude as the outer surface film. Nickel and molybdenum are enriched

in the alloy surface layer during active dissolution and passivation while iron is selectively

dissolved and chromium enriched as oxides and hydroxides in the outmost surface film. A

schematic picture of the passive film formed on stainless steels is presented in Fig. 2.

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alloy matrix

defects and pores

alloy surfacelayer, Ni, Mo

Fe-oxides/Cr-oxides

Stainless steel

alloy matrix

defects and pores

alloy surfacelayer, Ni, Mo

Fe-oxides/Cr-oxides

Stainless steel

Figure 2 Schematic picture of the composition of the outmost surface region on stainless steel.

Depending on area of application, demand for a certain appearance and/or corrosion

resistance, stainless steels are supplied with different surface finish. A large number of

different surface finishes are today available on the market such as reflecting mirror-like

surfaces and decorative patterned coloured surfaces. The most common surface finishes

available for cold-rolled stainless steel are present in Table 1 and described as they appear in

the EuroNorm and ASTM specifications [6].

Table 1 Common surface finishes on commercially available cold-rolled stainless steel grades

[6].

Euro norm ASTM

2E 1

Cold rolled, heat treated, mechanically descaled followed by pickling. Rough and dull.

2D 2DCold rolled, heat treated, pickled. Smooth.

2B 2BCold rolled, heat treated, pickled, skin passed.

Smoother than 2D.

2R BA

Cold rolled, bright annealed (may be skin passed).

Smooth, bright, reflective.

2H TR Work hardened. Bright.

Surface finishStandard

Process

The 2B surface is the most common surface finish and provides good corrosion resistance,

smoothness and flatness. The corrosion resistance and the cleanability is correlated to the

surface roughness and described in the European norm, EN10088 Part 2. Each surface finish

corresponds to a certain surface roughness, usually below 0.5 µm (expressed as the Ra-value).

However, as a result of variations between equipment used, mode of operation, and abrasive

medium used, surface finishes can vary somewhat between different production lines.

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1.2 Atmospheric corrosion of stainless steels

Since stainless steel is in use in a large variety of atmospheric environments and exposure

conditions, the general corrosion resistance of stainless steel in various media has traditionally

been thoroughly investigated [7]. The corrosion resistance is commonly described as a

relative measure of the corrosion protective properties of the passive film in various

environments and is closely related to the bulk alloy composition and prevailing exposure

conditions. The corrosion rate, on the other hand is a quantitative measure of the amount of

bulk metal that is oxidised per time unit and surface area, commonly expressed in mgm-2yr-1

or µmyear-1. Measurements of atmospheric corrosion rates of stainless steels are difficult to

perform, mainly because the corrosion process often is slow and of a local character (e.g.

pitting, crevice corrosion). As a result, relatively scarce data exist in the literature. Corrosion

rates in the range of 0.2-0.5 mgm-2year-1 and approximately 0.1 mgm-2year-1, have been

reported for grades 304 and 316 stainless steels, respectively [8]. These rates are significantly

lower compared to corrosion rates of other commonly used structural materials such as low-

alloyed steels, copper and zinc.

Long-term corrosion data for stainless steel exposed in marine and polluted locations [9-11]

have been compiled by NiDI (Nickel Development Institute) and used as guidelines for grade

selection in various environments [12]. Some of these studies report a negligible corrosion

rate for stainless steels containing more than 12 wt% chromium when exposed in industrial

and semi-rural environments, and a required chromium content above 15 wt% and alloying

with molybdenum when exposed in marine atmospheres [10]. The corrosion rate is generally

higher in marine or heavily polluted environments compared to rural locations, mainly as a

result of the presence of chlorides. Long-term exposures of stainless steel exposed for 20

years in South Africa and 5 years in Japan result in corrosion rates (based on mass loss)

between 0.025 and 0.93 µmyr-1 for grade 304 and between 0.025 and 0.279 µmyr-1 for grade

316 [12]. Corrosion rates less than 0.025 µmyr-1 have been reported for both grade 304 and

316 exposed during 15 years in a marine environment (Kure Beach, North Carolina, U.S.)

[11].

The atmospheric corrosion of stainless steel has also been evaluated in different environments

in terms of changes in e.g. visual appearance and corrosion resistance for different grades and

surface conditions [13-21].

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1.3 Metal release of alloy constituents from stainless steel during atmospheric exposure

The rate of metal release of alloy constituents is defined as the amount of metal that is

released from stainless steel per unit of surface area in a given time interval. Metals can be

released from stainless steel during atmospheric exposure as a result of a combination of

deposited corrosive species (e.g. gaseous and particulate pollutants), consecutive dry and wet

periods, and precipitation.

Few studies exist in the literature that report on quantitative data on release rates of alloy

constituents from stainless steel during atmospheric exposure. A combination of field and

laboratory investigations has recently been presented with the aim to provide long-term

annual release rates, to investigate the effect of specific parameters such as pH and rain

intensity in controlled laboratory investigations, and to study the environmental interaction of

corrosion-induced metal release of stainless steel [15, 21]. These investigations report total

metal annual release rates from grade 304 and 316 stainless steel exposed during four years in

the urban environment of Stockholm, Sweden (45º from the horizontal, facing south) between

0.2 and 0.7 mgm-2yr-1 for chromium, between 0.1 and 0.8 mgm-2yr-1 for nickel and between

10 and 200 mgm-2yr-1 for iron. The results show in contrast to the general experience,

somewhat higher metal release rates from grade 316 compared to 304 for each individual year

of exposure. Similar observations were made during a short-term investigation (5 months) in

Stockholm [22].

Somewhat higher release rates of alloy constituents from grade 316 compared to 304 were

also observed during detailed laboratory investigations focusing on the difference between

alloys and the pure metals [15, 23]. Results from this investigation are presented in Paper I.

All investigations show iron to be preferentially released and the release process to be time-

dependent with initially high rates (concentrations) that decrease to lower and more constant

rates (concentrations) with time and/or rain duration.

1.4 Corrosion of stainless steel in synthetic body fluids

Measurements of the corrosion rate of stainless steel exposed in synthetic body fluids are

difficult to find in the literature. Instantaneous corrosion rates have recently been presented

for stainless steel grade 303 and 304L by using electrochemical polarisation and impedance

measurements [24, 25]. This study involves measurements in PBS (phosphate buffered saline

solution), artificial sweat, ALF (artificial lysosomal fluid) and Gamble’s solution. The

instantaneous corrosion rate of grade 303 after one month of exposure was less than

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5 µgcm-2week-1 (1µgcm-2week-1 ↔ 520 mgm-2year-1) when exposed in artificial sweat and

Gamble’s solution, less than 10 µgcm-2week-1 in PBS and approximately 65 µgcm-2week-1

when exposed in ALF. The instantaneous corrosion rate of grade 304L was less than

5 µgcm-2week-1 for all fluids investigated.

1.5 Metal release of alloy constituents from stainless steel in synthetic body fluids

Metal release in a bulk solution is very different from metal release during atmospheric

exposure, since the former involves the continuous exposure to the bulk fluid of interest and is

intimately related to the solution volume whereas the latter a result of the occurrence of wet

and rainy periods and involves discontinuous exposures to a thin liquid film. The release

process and the corrosion process are furthermore not the same. The corrosion rate is usually

defined as the amount of oxidised metal per time and surface unit and includes both released

metals and metals in the adherent surface film whereas the metal release rate is defined as the

amount of released metal from stainless steel into a solution volume per time and surface unit.

Information on metal release rates from stainless steel in synthetic body fluids is scarce and

only few data are available for the pure metals that are used as alloy constituents in stainless

steels. Most studies in the literature are performed in bulk solution of stagnant flow

conditions. The available literature describes mainly dissolution studies of man-made fibres,

cobalt containing materials, indium phosphide etc. when exposed in synthetic body fluids [26-

35].

Stainless steel is widely used in a number of different applications, such as wristwatches,

piercing jewellery or implants. The stainless steel surface is then in direct contact with

different body fluids, such as sweat, saliva, and blood. In-vitro measurements are hence a

useful tool to obtain quantitative data on potential adverse effects of using stainless steel in

such applications.

Since contact dermatitis can be provoked by nickel, several investigations have focused on the

release of nickel from various stainless steel grades in artificial sweat, [25-28, 36, 37]. These

studies show the release rate of nickel from grade 304 and 316 to be less than

0.5 µgcm-2week-1 [28]. Release rates of chromium have also been reported [36].

Recent investigations of metal release of individual alloy constituents from stainless steel

grade 304L show total metal release rates of 0.36, 0.12, 0.4 and 1.98 µgcm-2week-1 when

exposed in PBS, artificial sweat, Gamble’s solution and ALF, respectively [25]. Iron was

preferentially released in all solutions.

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A compilation of literature data related to metal release studies in different synthetic body

fluids are given in Table 2.

Table 2 A compilation of literature references on investigations of metal release investigations in

synthetic body fluids.

Solution References Artificial sweat 25, 27, 28, 36-45 PBS 24, 25, 46 Lung fluid 33, 34 Gamble’s solution 25, 31, 32, 35, 61 Gastric fluid 33 Saline solution 26, 33, 43, 49, 57 Artificial saliva 26, 29, 30, 57 Organic acids 26, 66 Simulated Body fluids 56 Extracellular fluids 60 ALF/Cytosol 25, 60

1.6 Metal release of alloy constituents from stainless steel in different media

A number of investigations have been reported in the literature that focus on total release rates

of alloy constituents from stainless steels of different grades and corrosive solutions such as

sodium chloride solutions, seawater, sulphuric acid, acetic acid and heat-exchange water. A

compilation of references related to such investigations is given in Table 3.

Table 3 A compilation of literature on metal release related issues from stainless steel exposed in

different media.

Solution References Acids 28, 62, 63, 65 Chloride solutions 63, 64 Water/water solutions 63, 68 Sea water 69

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2 Experimental

The ultimate goal of this research investigation has been to provide quantitative data on metal

release rates of alloy constituents from stainless steel of different grade and surface finish

when exposed to different media (artificial rain and synthetic body fluids). In addition,

differences in metal release rates between stainless steel alloys and the pure metals have been

elucidated. The investigations involve the following analysis: i) total metal concentrations of

chromium, nickel, iron and molybdenum by using ICP/MS (Inductively Coupled

Plasma/Mass Spectrometry) and/or GF-AAS (Graphite Furnace-Atomic Absorption

Spectroscopy) to calculate release rates, ii) changes in surface film composition by means of

XPS, and iii) surface roughness using profilometry and EIS (Electrochemical Impedance

Spectroscopy). The experimental approach is schematically described below. Details for each

investigation are given in Papers I-III.

2.1 Investigated materials

A compilation of investigated materials (stainless steel, pure metals), synthetic media, and

exposure time-periods is given in Table 4. More specific data on the materials can be seen in

each paper.

Table 4 Compilation of materials, media and exposure time-periods investigated within each

research project.

Solution/Surface finish Material rain ALF Gamble Exposure time / h Paper no.

Grade 2205 2B, abr. 2B 8, 24, 48 168 II

Grade 201 shotblasted, abr.* shotblasted 8, 24, 48, 168 II

Grade 304 2B, abr*. 2B, abr*. 2B 8, 24, 48, 168; ½, 1, 2, 3, 4, 5, 6, 7, 8

I, II, III

Grade 310 2R, abr.* 2R 8 h, 24, 48, 168 II

Grade 316 2B, abr* ½, 1, 2, 3, 4, 5, 6, 7, 8 I

Grade 316L 2B, abr.* 2B 8, 24, 48, 168 II

Grade 409 2B, abr.* 2B 8, 24, 48, 168 II

Grade 430 2R, abr.* 2R 8, 24, 48, 168 II

pure Ni cold rolled, abr* ½, 1, 2, 3, 4, 5, 6, 7, 8 I

pure Cr electrolytic, abr* ½, 1, 2, 3, 4, 5, 6, 7, 8 I

pure Fe cold rolled, abr* ½, 1, 2, 3, 4, 5, 6, 7, 8 I

*abr. denotes mechanical abrasion to 1200P using SiC-paper.

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2.2 Experimental set up

2.2.1 Metal release investigations in artificial rain (Paper I)

All experimental work was performed at ambient laboratory conditions at KTH, Stockholm

using a rain chamber able to mimic outdoor rain events of varying intensity. All exposures

were performed on duplicate panels inclined 45º from the horizontal, using artificial rain with

a composition and pH representative of the southern and central part of Sweden [70]. The pH

of 4.4 is also representative of urban European conditions [71]. A rain intensity of 4 mmh-1

was selected for the investigation since most rain events in Sweden have an intensity between

2 and 6 mmh-1 [71].

The experimental set-up and procedure for performing the artificial rain investigations are

schematically illustrated in Fig. 3. A more extensive description of the experimental approach

is given in Paper I.

specimencollection box

compressed air

artificial rainwater

NO3-

SO42-

Cl-

NH4+

Na+

K+Mg2+

Ca2+

specimencollection box

compressed air

compressed air

artificial rainwater

NO3-

SO42-

Cl-

NH4+

Na+

K+Mg2+

Ca2+

as-receivedabraded

304, 316, Fe, Ni, Cr

8h rain 24 mm/h pH 4.4

pH 4.48h rain 14 mm/h

Dry period 40 daysXPS ICP/MS

as-receivedabraded

304, 316, Fe, Ni, Cr

8h rain 24 mm/h pH 4.4

pH 4.48h rain 14 mm/h

Dry period 40 daysXPS ICP/MS

Figure 3 Experimental set-up and procedure for performing the artificial rain investigation

(Paper I).

2.2.2 Metal release investigations in synthetic body fluids (Paper II+III)

Immersion exposures were performed in two different synthetic body fluids: Gamble’s

solution that mimics the interstitial fluid of the deep lung (Paper II), and artificial lysosomal

fluid (ALF) that mimics the acidic Cytosol solution of cell macrophages (Paper II and III).

Cytosol is the internal fluid of the cell [72] where the main part of the cell metabolism occurs

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and in which the cell organelles dwell. The Cytosol fluid contains 20-30% proteins that

control the metabolism.

The difference between the two fluids can be described in the following way: when e.g. a

small particle (∅ <3 µm) is inhaled, it will end up deep down in the lung compartment, an

environment simulated by the Gamble’s solution of nearly neutral acidity (pH 7.4). White

blood cells will then engulf the particle, which then will be exposed to a more acidic

environment (pH 4.5), simulated by ALF. All experiments have been performed on massive

sheet in order to define a limited number of stainless steel grades that would be most relevant

for similar investigations of stainless steels in the potentially inhalable powdered form.

Further details are given in Paper II.

A schematic description of the experimental set-up is given in Fig. 4. A description of the

surface finishes studied in paper III is shown in Fig. 5. Further experimental information is

given in Papers II and III.

2205,201, 304, 310,

316, 409, 430

• 8h, 24h, 48h, 168h• dark, 37°C

• surface area /solution volume ≈ 1 cm-1

RBS 2% 50°Cultra pure water

XPS

EIS (304)profilometry

ALF pH 4.5

ICP/MS

Gamble’spH 7.4

GF-AAS

as-received abraded

2205,201, 304, 310,

316, 409, 430

• 8h, 24h, 48h, 168h• dark, 37°C

• surface area /solution volume ≈ 1 cm-1

RBS 2% 50°Cultra pure water

XPS

EIS (304)profilometry

ALF pH 4.5

ICP/MS

ALF pH 4.5

ICP/MS

Gamble’spH 7.4

GF-AAS

Gamble’spH 7.4

GF-AAS

as-received abraded

Figure 4 A schematic description of the experimental set-up for metal release investigations in

synthetic body fluids (Paper II and III).

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Grade 304

2B

2D

2R

Heat treated, cold rolled, pickled, skin passed

Heat treated, cold rolled, pickled

Heat treated, bright annealed

Grade 304

2B

2D

2R

Heat treated, cold rolled, pickled, skin passed

Heat treated, cold rolled, pickled

Heat treated, bright annealed

Figure 5 Different surface finishes of grade 304 studied in paper III.

2.3 Analysis of total metal concentrations

2.3.1 Inductively coupled plasma-mass spectrometry (ICP/MS)

By this technique low concentrations (µgL-1) of total metal content in fluids can be analysed.

The detection limit was 60, 2.5, 4.0 and 1.0 µgL-1 for Fe, Ni, Cr and Mo, respectively

ICP/MS is a high temperature (8000°C) technique where a plasma is formed by argon gas

passing a radio frequency field, in which the argon gas molecule is partly ionised. The test

solution is introduced into the plasma that ionises the metal atoms, can pass a dynamic

reaction cell (gas reactions to avoid interferences) and finally reaches a mass spectrometer.

The dynamic reaction cell, however, was not used for any measurements within this thesis.

All measurements were performed at the Environmental Research Laboratory, Swedish

University of Agricultural Sciences, Umeå, Sweden

2.3.2 Graphite furnace atomic absorption spectroscopy (GF-AAS)

Sub-ppb concentrations of metals can be analysed with GF-AAS with a detection limit of 0.3,

0.5, 0.25 and 0.25 µgL-1 for Fe, Ni, Cr and Mo, respectively.

The test solution is introduced into a graphite tube and heated to approximately 2000°C

(depending on element). The element of interest is atomised in a vapour through which light

of a specific wavelength from a cathode lamp passes. This results in a decreased intensity (in

proportion to the elemental concentration) that is isolated with a monochromator and finally

measured by a detector.

All measurements were performed at the Swedish Pulp and Paper Research Institute,

Stockholm, Sweden.

13

2.4 Analysis of surface film composition and surface roughness

2.4.1 X-ray photoelectron spectroscopy (XPS)

This technique allows measurements of the chemical state of elements in the outermost

surface film (1-5 nm).

A solid surface sample, irradiated by high-energy photoelectrons, emits inner shell electrons

of atoms. All electrons, which have a lower binding energy than the exciting photoelectrons,

can be ejected and analysed in terms of their kinetic energy, from which the binding energy

can be calculated. Each element has its own characteristic spectrum of binding energies,

which are slightly shifted, depending on chemical state [73, 74].

2.4.2 Profilometry

This technique allows measurements of the surface area including all peaks, valleys and other

surface features with a lateral resolution of 1.1 µm.

Parallel white light is focused perpendicularly onto the surface of a reference sample and a

test sample. Optical path differences are then achieved due to differences in height between

the reference sample and the test sample. Analysis of the intensity and phase of the complex

interference pattern results in information on shape and surface roughness of the test sample

surface [75].

All profilometry measurements were performed at the Institute for Surface Chemistry,

Stockholm, Sweden.

2.4.3 Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance measurements can be used as a relative measure of the

electrochemically active surface area, which is proportional to the capacitive response of a

metal surface-electrolyte interface [76].

The equivalent circuit used for fitting EIS results is shown in Fig. 6. Re represents the

electrolyte resistance, Rp the charge transfer resistance and C the interfacial capacitance.

14

Re

Rp

C

Re

Rp

C

Figure 6 Schematics of equivalent circuit used for modelling of the metal surface-electrolyte

interface. Re represents the electrolyte resistance, Rp the charge transfer resistance and C

the interfacial capacitance.

Rp is inversely proportional to the geometric surface area (eq. 1) and the interfacial

capacitance is directly proportional to the effective surface area (eq. 2). ε is the dielectric

constant of the medium and ε0 is the dielectric permittivity of vacuum, A is the geometric

area, Aeff (eq. 3) is the effective area and d is the distance between the charged plains at the

interface (eq. 2).

Rp ∝ 1

ARp ∝ 1

A1A (eq. 1)

C = εε0rAd

C = εε0rAd (eq. 2)

Aeff

Ar =

Aeff

Ar =

(eq. 3)

15

3. Summary of results

3.1. Why is it important to provide metal release data of alloy constituents from stainless

steels?

A stategy for the handling of chemicals was adopted in 2001 within the legislative framework

of the European comission [77]. The main objective of this strategy is to ensure a high level

of protection for human health and the environment. The policy is often referred to as

REACH (Registration, Evaluation, Authorization of CHemicals) and requires the

manufacturer and industry to demonstrate that all chemicals used within their production are

safe to use and do not impose a threat to or impact on the environment. A chemical substance

is defined as dangerous if the ingredients of the product are included in the List of Dangerous

Substances. In addition, the policy does not take into account the fact that metal alloys, e.g.

stainless steel, often possess fundamentally different e.g. mechanical, physical, chemical, and

toxicological properties compared to individual alloy constituents. Stainless steel is an iron-

based alloy which contains varying amounts of metallic (e.g. Cr, Ni, Mo) and non-metallic

elements (e.g. C, N, S).

Since stainless steels are used in a wide variety of applications ranging from external

constructions (e.g. buildings, bridges) to biomedical applications (e.g. surgical implants,

orthodontic wires) and jewellery (e.g. wristwatches, body piercing applications) it may come

into contact with a wide range of media including rain water and different body fluids

(investigated within this thesis). Information on the corrosion resistance of stainless steel

exposed in different media is well documented; however there is a considerable lack of

knowledge on release rates of alloy constituents of stainless steel and the pure metals into

different media. Since the precautionary principle often is used when reliable scientific data

are unavailable, the need for reliable and scientifically sound data is evident for sustainable

decisions and legislative actions towards the use of metals and alloys.

3.2 How does the degree of alloying influence the metal release rate?

Numerous investigations have shown the corrosion resistance of stainless steel to be improved

by an increasing chromium bulk content (> 13wt%) and/or alloying with metals such as nickel

and molybdenum [7]. This beneficial effect was also seen on released alloy constituents from

stainless steel when exposed in one of the investigated synthetic body fluids, ALF (artificial

16

lysosomal fluid), for one week. The results are illustrated in Fig. 7 (left) showing the total

weekly metal release rate (Cr+Ni+Fe+Mo) from seven different grades of stainless steel of

varying bulk content of chromium (11-24 wt-%), nickel (0-19 wt-%) and molybdenum (0-3

wt%). The highest release rate was seen for the ferritic steel grade 409, being the least alloyed

stainless steel grade (11 wt-% Cr and <0.12 wt-% Ni) investigated, and the lowest release rate

for grade 310, being the most alloyed grade (24 wt-% Cr and 19 wt-% Ni). The total weekly

metal release rate increased with decreasing chromium content of the austenitic grades.

0

1

2

3

4

5

310 2205 304 201 316L 430 409

decreasing chromium content

Tot

al m

etal

rel

ease

/µg

cm-2

wee

k-1

A D A A A F F

24% 22% 18% 18% 17% 16% 11%

Stainless steel grade

0

1

2

3

4

5

310 2205 304 201 316L 430 409


Tot

al m

etal

rel

ease

/µg

cm-2

wee

k-1

A D A A A F F

24% 22% 18% 18% 17% 16% 11%


0

0,2

0,4

0,6

0,8

1

310 2205 304 201 316L 430 409

Cr/

(Cr+

Fe)



0

0,2

0,4

0,6

0,8

1

310 2205 304 201 316L 430 409

Cr/

(Cr+

Fe)


Stainless steel grade Figure 7 Total weekly metal release rates of the sum of Cr, Ni, Fe and Mo from different grades of

stainless steel (left) and chromium content in the surface film (right) after exposure in

ALF for one week. The grades are arranged according to the decreasing chromium bulk

content (wt%, given in each pile). Abbreviations A, D and F denote, austenitic, duplex

and ferritic stainless steel grades.

Investigations on the chromium enrichment in the surface film showed an inverse effect on

the total metal release rate. However, the degree of chromium enrichment of the surface film

could not unambiguously be correlated to the degree of alloying, Fig. 7 (right).

In all, it can be concluded that total weekly release rates of alloy constituents are related to the

degree of alloying and decreased according to the following sequence: grade 409 >> grade

430 > grades 316L ≈ 201 ≈ 2205 ≈ 304 > grade 310. Also other parameters such as surface

finish, surface film thickness, composition and presence of defects, influence the rate of metal

release. More details are given in Paper I.

17

3.3 Is the metal release process of alloy constituents time-dependent?

Previous investigations have shown release rates of copper and zinc from various

commercially available construction materials to be time-dependent, with initially high

release rates (first flush) that decrease to lower and relatively constant rates (steady-state) with

time and/or increasing rain volume [78-81]. Similar observations were observed for stainless

steel, both when exposed to a continuous rain event (thin film conditions) and during

immersion experiments (bulk conditions). This is exemplified in Fig. 8 showing changes in

metal release rates of chromium from stainless steel grade 316L during an 8-hour continuous

rain event (left) and immersion for 168 hours in ALF (artificial lysosomal fluid) (right). All

alloy constituents released from stainless steel show similar time-dependence. The large

difference in metal release rates between the two media is mainly attributed to differences in

exposure conditions. The metal release process is often rate-limiting under thin film

conditions (< 10 µm), since oxygen is readily transported to the cathodic site, whereas the

transport of oxygen may be the rate-limiting step under bulk conditions [8].

The time dependence is related to the gradual formation of a more corrosion resistant passive

surface film. Compositional measurements with x-ray photoelectron spectroscopy (XPS)

showed a gradual chromium enrichment with time of the surface film on stainless steel for all

grades and exposure media.

0

1

2

3

4

5

6

½ 1 2 3 4 5 6 7 8

Rel

ease

rate

/ ngc

m-2

h-1

Rain duration / h

0

1

2

3

4

5

6

½ 1 2 3 4 5 6 7 8

Rel

ease

rate

/ ngc

m-2

h-1

Rain duration / h

0

0,2

0,4

0,6

0,8

1

8 24 48 168

Immersion time / h

Rel

ease

rate

/ µgc

m-2

wee

k-1

0

0,2

0,4

0,6

0,8

1

8 24 48 168

Immersion time / h

Rel

ease

rate

/ µgc

m-2

wee

k-1

Figure 8 The time-dependence of metal release of alloy constituents from stainless steel

exemplified for chromium and grade 316L exposed to a thin film of rainwater impinging

a surface inclined 45º from the horizontal (left) and immersed in the synthetic body fluid

ALF (right).

In all it can be concluded that stainless steel shows time dependent metal release rates of alloy

constituents in both rain water and synthetic body fluids. The decrease in metal release rates

18

was attributed to a gradual enrichment of the chromium content of the surface film on all

grades.

More details are given in Paper I (rain water) and Paper II (synthetic body fluids).

3.4 Is the release rate the same for all alloy constituents?

The passive film on stainless steel is composed of an outer layer mainly of chromium oxide or

hydroxides and an inner layer of both chromium- and iron oxides. Nickel, molybdenum and

iron are primarily present in a thin layer below the passive film, see Fig. 2 (Introduction).

Although chromium is present in the outermost layer of the passive film and the other alloy

constituents below this barrier, chromium shows the lowest release rates in all media

investigated.

Differences in metal release rates of individual alloy constituents are illustrated for grade 316

exposed to a continuous rain event (Fig. 9, left) and for grade 316L exposed in ALF (Fig. 9,

right).

00,5

11,5

22,5

33,5

4

Ni Cr Fe MoRel

ease

rate

/ ng

cm-2

wee

k-1

00,5

11,5

22,5

33,5

4

Ni Cr Fe MoRel

ease

rate

/ ng

cm-2

wee

k-1

0

0,2

0,4

0,6

0,8

1

1,2

1,4

Ni Cr Fe MoRel

ease

rate

/µg

cm-2

wee

k-1

0

0,2

0,4

0,6

0,8

1

1,2

1,4

Ni Cr Fe MoRel

ease

rate

/µg

cm-2

wee

k-1

Figure 9 Release rates (steady-state conditions) of individual alloy constituents from stainless steel

grade 316 exposed to a continuous rain event (left) and grade 316L exposed in ALF

(right).

Iron is preferentially released compared to nickel, chromium and molybdenum for all grades,

surface finishes and media investigated. The release of nickel, iron and molybdenum suggests

the presence of defects and inhomogenieties within the passive film, which facilitate transport

of these metals from the alloy surface layer through the film. Nickel is enriched in the alloy

surface layer during active dissolution and passivation of stainless steel at the same time as

iron and chromium are depleted. As a result, iron is selectively released and chromium

enriched in the passive film. More details are given in Papers I, II and III.

19

In all, it can be concluded that release rates of iron were significantly higher than those of

chromium, nickel and molybdenum for all stainless steel grades, surface finishes and media

investigated.

3.5 Are laboratory investigations relevant for real exposure scenarios?

Field exposure in natural atmospheres involves the combined effect of a large number of

interacting parameters that may affect the metal release process of alloy constituents from

stainless steel, whereas laboratory investigations enable the possibility to elucidate the effect

of a specific parameter. The possibility to successfully mimic natural outdoor conditions in

the laboratory has recently been presented in a combined field and laboratory investigation

focusing on metal release rates of alloy constituents from stainless steel grades 304 and 316

[15, 20].

A comparison between results on release rates of chromium, nickel, and iron from stainless

steel grades 304 and 316 during continuous rain events in the laboratory (Paper I) and outdoor

annual release rates from the same stainless steel grades exposed during four years in the

urban environment of Stockholm [82], Sweden, exhibits consistency. All laboratory

measurements were performed using a rain pH (4.4), a composition resembling the central

and southern part of Sweden and a rain intensity of 4 mmh-1. Using steady state

concentrations of released chromium, nickel and iron and the assumption of altogether 100

hours of rain during a year [20], laboratory data resulted in annual release rates between 0.1

and 0.2 mgm-2yr-1 for both chromium and nickel and between 2 and 10 mgm-2yr-1 for iron.

These rates are somewhat lower, but of the same order of magnitude as observed release rates

obtained in natural rain (Cr: 0.2-0.7 mgm-2yr-1, Ni: 0.1-0.8 mgm-2yr-1, Fe: 10-200 mgm-2yr-1)

[21]. The differences are expected when considering that the outdoor exposure situation is

much more complicated compared to the laboratory exposure with a single rain event of

constant rain composition, intensity and pH.

Because of practical and ethic considerations in-vivo metal release investigations from

stainless steel are not possible to perform. Such investigations require tests in-vitro, even

though the results cannot fully be verified with in vivo measurements. Another drawback is

the chemical composition of fluids of the human body that cannot be simulated in a totally

realistic way. Synthetic body fluids that contain the main components of natural body fluid

can, however, be used for comparative purposes and indicate the magnitude of metal release.

20

In all, it can be concluded that laboratory in-vitro investigations are important to elucidate the

metal release process in various media.

3.6 Are metal release rates of alloy constituents from stainless steels equal to metal

release rates from pure metals?

Two grades of stainless steels (grades 304 and 316) and the corresponding pure metals were

exposed to identical simulated rain events in order to examine differences in metal release

rates. The results are illustrated in Fig. 10 for grade 316 and the pure metals.

Release rates of iron and nickel from the pure metals were substantially higher than the

release rate from the stainless steel alloy. The reason is that both nickel and iron is

predominantly present in the alloy surface layer adjacent to the chromium rich passive film,

which act as a barrier for transport of these metals through the film. Iron can also be released

from the passive film, since this film consists of both chromium and iron

oxides/oxyhydroxides. Pure iron and nickel form, on the other hand, relatively defective and

poorly adherent surface films during atmospheric exposure, which enable metals to be more

easily released. Comparable release rates of chromium were determined for the steel alloys

and the pure metal as a result of the presence of a passive film containing chromium oxides

and hydroxides on all surfaces, acting as a barrier for corrosion and metal release. Similar

results were seen for grade 304.

0

1

2

3

4

5

Ni Cr Fe

Grade 316Pure metal

/10

/40

rele

ase

rate

/ng

cm-2

h-1

0

1

2

3

4

5

Ni Cr Fe

Grade 316Pure metalGrade 316Pure metal

/10

/40

rele

ase

rate

/ng

cm-2

h-1

Figure 10 A comparison between release rates of nickel, chromium and iron from grade 316 and the

pure metals exposed to 8 hours of continuous rain (pH 4.3, intensity 4 mmh-1).

In all, it can be concluded that the presence of a chromium rich passive film on stainless steel

substantially reduces the release rate of iron and nickel compared to the release rate from the

pure metals. However, similar release rates of chromium were seen for stainless steel alloys

and pure chromium. More details are given in Paper I.

21

3.7 Can metal release rates from stainless steels be estimated based on the nominal

alloy composition and metal release data from pure metals?

Estimates of the potential metal release from stainless steel are often made to assess the

potential risk of adverse effects arising from exposure to the metals contained in stainless

steel. Due to lack of understanding of the difference between an alloy and a pure metal, such

estimates are usually based on the environmental effect of the pure metal and the nominal

composition of the alloy without considering the fact that alloys and pure metals show

significant differences in, e.g., corrosion resistance and mechanical properties.

A comparison between actual release rates of alloy constituents and calculated rates based on

release rates from the pure metals and the nominal alloy composition was therefore made to

illustrate the incorrectness in performing such calculations and to illustrate the large

discrepancy between measured and calculated data. The results are depicted in Fig. 11 for

stainless steel grade 316 and the pure metals exposed to identical simulated rain events.

Calculated release rates of nickel and iron are orders of magnitude higher compared to

measured rates, whereas slightly lower measured rates compared to calculated rates are seen

for chromium. Similar observations were seen for grade 304. The comparison illustrates the

importance of using real metal release data from the actual steel and from its pure metals.

More details are given in Paper I.

0

0,5

1

1,5

2

2,5

3

Fe Ni Cr

/500

rele

ase

rate

/ng

cm-2

h-1

/2

316 measured316 calculated

0

0,5

1

1,5

2

2,5

3

Fe Ni Cr

/500

rele

ase

rate

/ng

cm-2

h-1

/2

0

0,5

1

1,5

2

2,5

3

Fe Ni Cr

/500

rele

ase

rate

/ng

cm-2

h-1

/2

316 measured316 calculated316 measured316 calculated

Figure 11 Measured and calculated release rates of alloy constituents from grade 316 exposed to a

continuous rain event for eight hours (pH 4.3, 4 mmh-1).

In all, it can be concluded that calculations of release rates, based on nominal alloy

compositions and release rates from pure metals, show erroneous results that substantially

overestimate actual release rates of nickel and iron from stainless steel, but not of chromium.

22

3.8 What kind of difficulties can be encountered during in-vitro exposures in synthetic

body fluids?

One major problem evolved during exposures of stainless steel samples in the near neutral

(pH 7.4) Gamble's solution (interstitial fluid of the deep lung). All stainless steel surfaces

formed a yellowish and/or bluish surface film, predominantly composed of phosphate and

calcium as observed with XPS, already within a few hours of exposure. Similar experiences

have previously been reported [25, 31]. The acidity decreased at the same time from pH 7.4 to

well above 8. This decrease in acidity and subsequent surface deposition had a inhibitive

effect on the metal release rate. However, such changes are not acceptable for providing

reliable and reproducible data.

The effect of solution volume to vessel volume ratio (a measure of the volume of air inside

the test vessel), and the effect of different ways of sealing the lids of the vessels (to avoid air

from entering the vessels) are presented in Fig. 12 for LDPE (low density polyethylene)

vessels. All tests were performed at 37ºC in Gamble's solution without any stainless steel test

samples. A small volume of air inside the test vessel resulted in the smallest change in pH. No

significant effect was seen between different ways of sealing. Tests with HDPE (high density

polyethylene) showed similar results.

7

7,5

8

8,5

solution volume /vessel volume

pH

0.43 0.61 0.78 1

closed closed + plastic

closed + teflon tapeclosed + teflon tape + plastic

Starting pH = 7.43

7

7,5

8

8,5

pH

0.43 0.61 0.78 1



Starting pH = 7.43

7

7,5

8

8,5

solution volume /vessel volume

pH

0.43 0.61 0.78 1



Starting pH = 7.43

7

7,5

8

8,5

pH

0.43 0.61 0.78 1



Starting pH = 7.43

Figure 12 Effect of solution volume to vessel volume ratio and type of sealing of the lids of the test

vessel (LDPE) exposed for 18 hours in Gamble’s solution.

Comparative studies on changes in pH during exposure were performed in test vessels of

LDPE and glass using the same solution volume to vessel volume ratio. Test vessels of glass

were found to have a significantly smaller effect on the acidity of Gamble's solution compared

to test vessels of LDPE. Exposures were then performed with glass vessels and stainless steel

23

samples at different exposure temperatures (36°C and 39°C) and solution volume to vessel

volume ratios (0.45 and 0.91). The results are presented in Table 5 showing a small volume of

air inside the test vessel and a temperature below 39°C to have the lowest influence on the

acidity during exposure.

Table 5 Effect of solution volume/vessel volume ratio and exposure temperature of solution pH

for stainless steel samples exposed for one week in Gamble’s solution using glass vessels.

Temperature / °C solution volume/ vessel volume

pH

36 0.45 8.0 36 0.91 7.6 39 0.45 8.0 39 0.91 7.7

In all, it can be concluded that the pH stability of Gamble's solution is strongly influenced by

the choice of test vessel material, the ratio between the solution volume and the vessel

volume, and on temperature. Test vessels of glass, small volumes of air inside the vessels and

a temperature of 37±1°C resulted in a small change in pH, no visual changes in surface

appearance and minor adsorption of calcium and phosphate on the surface of stainless steel

exposed in Gamble's solution.

3.9 Does the surface finish of stainless steels have any effect on metal release rate?

Stainless steels are produced with different surface finish depending on e.g., area of

application, prevailing environmental conditions, demand on corrosion resistance and/or

appearance. The surface finish may influence surface properties, such as surface roughness,

effective surface area and surface composition, which may have an effect on metal release

rate of alloy constituents from stainless steels.

The effect of surface finish was investigated for grade 304 of surface finish 2D (cold rolling,

heat treatment, pickling), 2B (cold rolling, heat treatment, pickling, skin passing) and BA

(cold rolling, bright annealing) exposed to the synthetic body fluid ALF. The results are

illustrated in Fig. 13 showing the total weekly metal release rate to decrease with the

following sequence of surface finishes: 2D > 2B ≈ BA. These differences were most closely

correlated to the electrochemically active surface area, measured by means of electrochemical

impedance spectroscopy, (EIS). More details are given in Paper III.

24

00,20,40,60,8

11,21,4

2D 2B BAT

otal

rel

ease

rat

e /µ

gcm

-2w

eek-1

Figure 13 Total weekly release rates of alloy constituents (Cr+Ni+Fe) from stainless steel of

different surface finish exposed in ALF for one week.

In all, it can be concluded that stainless steel of different surface finish exhibit slight

differences in metal release rates under actual exposure conditions.

3.10 How can we use the generated metal release rates?

Release rate data generated within this licentiate thesis represent information that can be used

for a first evaluation of the potential environmental risk of adverse effects of stainless steel in

a given application. It should be remembered, however, that a more comprehensive exposure

assessment requires not only data on release rates of iron, chromium, nickel and molybdenum

from stainless steel, but also information regarding chemical speciation of these elements in a

given environment. The focus of this work, however, was on metal release rates rather than on

metal chemical speciation.

Nevertheless this work exhibits very low release rates of chromium, nickel and molybdenum

(when measured) in the investigated exposure situations, with metal concentrations usually far

below those recommended for health issues. A recent four-year urban field exposure with

stainless steel grades 304 and 316 presents median release concentrations of chromium, nickel

and iron of 0.5, 0.7 and 140 µgL-1, respectively [20]. Laboratory exposures of the same grades

with synthetic rainwater result in concentrations less than 1.2 µgCrL-1, <0.7 µgNiL-1, and <15

µgFeL-1 during steady-state conditions of a single rain event (Paper I). It should be noted that

the concentrations of chromium and nickel are significantly lower than WHO health

recommendations for drinking water (<50 µg CrL-1, <20 µg NiL-1). The only recommendation

given for iron (< 300 µgL-1) is related to staining of laundry and drinking of well water [83].

25

Low metal concentrations (µgL-1) were also obtained during exposure in synthetic body

fluids. Higher concentrations were obtained in the more acidic fluid of ALF compared to the

near neutral Gamble's solution (Paper II).

Total release rates of alloy constituents from stainless steel exposed in artificial rainwater

(Paper I) and synthetic body fluids (Paper II) are compiled in Table 6. In order to increase the

comprehension of the very low magnitude of these rates, they have been recalculated to the

corresponding decrease in surface thickness of the stainless steel (expressed as numbers of

mono-layers per week). However, it should be noted that the release process and the corrosion

attack on stainless steel largely occur locally over the surface. A release rate of one

µgcm-2week-1 corresponds to the loss of one monolayer of the stainless steel surface every

two days.

Table 6 Total metal release rates from stainless steel exposed in artificial rainwater and synthetic

body fluids (ALF, Gamble’s solution) expressed as weekly rates and recalculated to

weekly removal of number of monolayers of stainless steel.

Artificial rain ALF Gamble’s solution

Total release rate / µgcm-2week-1 0.73 1.3 0.075

Corresponding number of mono-layers removed per week 2.6 4.6 0.3

In all, it can be concluded that a first evaluation of the potential of adverse effects of released

alloy constituents from stainless steel can be made from the magnitude of released metal

concentrations and/or rates. Release rates of alloy constituents from stainless steel are very

low and correspond to the removal of a few monolayers of stainless steel every week.

Released metal concentrations in rainwater are well below WHO health recommendations for

drinking water.

26

4. Main conclusions Different grades of stainless steel have been exposed to artificial precipitation and to synthetic

body fluid of varying composition and pH in order to assess release rates of primarily iron,

nickel, chromium and molybdenum into the investigated environments.

The following main conclusions could be drawn:

• Total release rates from all investigated stainless steels into synthetic body fluid

decreased with increasing degree of alloying in the following order: grade 409 >>

grade 430 > grades 316L ≈ 201 ≈ 2205 ≈ 304 > grade 310.

• Metal release of alloy constituents from stainless steel into rainwater and synthetic

body fluids was initially high and decreased with exposure time. The decrease was

attributed to a gradual increase of chromium in the passive film.

• Iron was preferentially released from all grades independent of surrounding media.

Nickel, chromium and molybdenum exhibited significantly lower, but similar, release

rates, also when considering the bulk content of these elements.

• Release rates of iron (2 µgcm-2week-1) and nickel (0.08 µgcm-2week-1) from stainless

steel grades 304 and 316 exposed to artificial precipitation were much lower than

corresponding rates for the pure metals (750 µgcm-2week-1 released Fe and 15

µgcm-2week-1 released Ni), whereas chromium exhibited similar release rates from

stainless steel and the pure metal (0.1 µgcm-2week-1).

• Calculations of release rates, based on nominal alloy composition and release rates

from the pure metals, significantly overestimate measured release rates of iron and

nickel from stainless steel, but not of chromium.

• Release rates of all alloy constituents into different synthetic body fluids exhibited a

marked pH-dependence and decreased significantly with increasing solution pH.

27

• Surface finish had only a slight influence on the total release rate into synthetic body

fluid, and varied in the following release rate order: 2D > 2B ≈ BA. This order was

mainly attributed to the electrochemically active surface area generated by the surface

finish treatments.

• Release rates in artificial rain were found to concur with field data from previous

studies.

• In-vitro investigations in synthetic body fluid are useful for predicting metal release

processes in various body fluid media. Care has to be taken, however, when using

Gamble’s solution because of a marked sensitivity towards solution temperature,

vessel material and volume of air inside the vessel.

5 Acknowledgements I wish to thank several people for making this thesis possible:

• Professor Christofer Leygraf for giving me the possibility to work with interesting

projects and for providing a working atmosphere that makes you happy when you go

to work.

• Assistant Professor Inger Odnevall Wallinder, the best supervisor possible, for all your

time and support during the experimental work and the writing of this thesis.

• All my brilliant colleagues, past and present, at the Division of Corrosion Science,

especially my roommates Anna and Klara.

• Pat Koundakjian, Staffan Malm and Tony Newson at Eurofer and ISSF for all your

help.

• Eurofer and International Stainless Steel Forum (ISSF) are greatly acknowledged for

their financial support.

• Bengt Andersson and Birgitta Olsson at SLU, Umeå, for helping me analyse my

samples.

• My parents, Anita and Carl-Johan, for all your support and encouragement during my

undergraduate studies and for continuing to support me during my graduate studies. It

would not have been possible without you.

• All my friends at Kalvsvik for providing the perfect place for recreation.

28

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