Outokumpu Stainless

Dear Reader

Most of us recognise words that are as thick as thieves, e.g. in an old hit song by Bing Crosby (or was it Frank Sinatra?), i.e. “Love and Marriage, Horse and Carriage”, and from our world, the world of metallurgy, “Pickling and Passivation”. They just stick together.

It is time for a change now, a divorce, maybe not necessarily between “love and marriage”, but surely between “pickling and passivation”.

This issue of Acom clearly shows that an extra passivation is not required if the stainless steel has been properly pickled resulting in a clean and shiny stainless steel surface. Any extra passivation will just imply extra costs without, with a few rare exceptions, contributing to improved corrosion resistance.

“Passivation” could be justified as a cleaning operation if further fabrication after the pickling has contaminated the stainless steel surface, but then the purpose is to clean the surface, not to passivate it.

Enjoy the reading, think, and start reducing your fabrication costs!

Yours sincerelyJan OlssonTechnical editor of Acom

Passivation treatment of stainless steel

acom4 - 2004A corrosion management and applications engineering magazine from Outokumpu Stainless

Passivation Treatment of Stainless Steel Lena Wegrelius and Birgitta SjödénOutokumpu Stainless AB

AbstractThe effect of different passivation treatments on corrosion performance of the stainless steel grade 1.4404 has been studied. Focus was directed towards the durability of the passivation treatment. The results from the corrosion tests that have been performed directly after passivation show a slight improvement of the corrosion resistance after treatment with nitric acid. However, the result from the duration test shows that the passive film adjusts itself according to the environment and reaches an equilibrium state with optimum properties after approximately one day, independent of previous passivation procedure.

The result in this study shows that passivation is rarely needed for improved corrosion resistance. The optimum corrosion properties will be reached anyhow within one day or two. On the other hand, passivation is an effective way to clean the stainless steel surface from contaminants such as shop dirt or particles of iron from different fabrication operations, which otherwise could form rust or act as initiation sites for corrosion.

IntroductionManufactures and users of modern food and pharmaceutical processing equipment very often demand a final step of surface treatment, i.e. passivation, in order to achieve a product with acceptable visual appearance and high corrosion resistance. The passivation process removes chips and “free iron” contaminations left behind on the surface from different fabrication operations. These contaminants are potential corrosion sites that may result in premature corrosion and ultimately in deterioration of the component if not removed.

Passivation with nitric acid enhances the chromium fraction in the passive film. The main mechanism for this process is selective dissolution of predominantly iron, which results in an increased corrosion resistance [1]. Although several investigations have found that passivation treatments usually have a beneficial effect on the corrosion resistance, very little is known about the passivation effect with time. For how long will the effect last? What will happen with the passivated surface when it is subject to a long-term corrosive media? Is there a need for renewing the passivation treatment after a certain time? Such kinds of questions still have to be answered.

Several passivation treatments may be used as described in ASTM A 380 and in ASTM A 967. The choice of surface treatment for a certain application is often based on tradition rather than insight. Although there are many options of passivation solutions, the overwhelming choice is still the nitric acid based solutions. Nitric acid has the disadvantage of being hazardous from health and environmental point of view. Another option is passivation with citric acid, a milder alternative to nitric acid. Even though citric acid is milder to stainless steel it gives acidic waste and thereby still causes environmental problems.

A passivation solution recently widely advertised is the use of chelating agents. It is claimed that these comprise extremely versatile, highly effective compounds, which complex and remove a variety of metallic ions that would otherwise adversely affect the corrosion resistance of the alloy. Chelat comes from the Greek word “chela” meaning claw. Its function is explained in that way that a ring structure is formed around a contaminant metallic ion that is removed and strongly held. The chemicals bond of the metallic ion are

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fixed within and inactivated by the ring structure of the chelat surrounding it. It cannot function as a metallic element any longer and is removed by the chelating agent [2].

Commonly used chelats are polyfunctional organic carboxylic acids, such as EDTA and citric acid, with salts containing hydroxyl and amine constituents. However, a forthcoming problem with at least some of the chelat agents is that the wastewater treatment plants are not able to decompose these sequestering agents, which implies that heavy metals are transported through the wastewater plant untreated directly into nature. Restrictions regarding this kind of waste deposits are therefore under discussion.

Recently, there has been research performed to develop alternative processes and solutions that are both user and environmentally friendly, yet equally effective. One of those is Avesta Welding’s new product, FinishOne 630, composed by dilute hydrogen peroxide solution with additions of stabilizing agents.

The aim of this investigation was to compare different passivation treatments with the main focus directed towards the durability and continuing high corrosion performance, especially in the long run.

ExperimentalMaterial and Surface PreparationThe stainless steel studied was the austenitic grade EN 1.4404 (316L), 17.2% of Cr-10.2% of Ni-2.1% of Mo-balance Fe, per cent by weight.

The specimens were sawed, cut and polished from 3mm cold rolled and pickled (2B-finish) sheet material to sizes and finishes according to the different test/analysis methods, see table 1. The specimens used in the electrochemical experiments were mounted in an epoxy resin.

Test/Analysis method Investigation Dimensions Finish (mm)

ASTM G 150 CPT1 60 x 60 P320 wet

ASTM G 48 CCT 2 25 x 50 P120 dry

Electrochemicalmeasurements Open circuit potential 10 x 10 P320 wet

X-ray PhotoelectronSpectroscopy, (XPS) Surface chemistry 10 x 10 Diamond paste 1 µm

1. Critical Pitting corrosion Temperature, °C.2. Critical Crevice corrosion Temperature, °C.

Dimensions and surface finishes of the specimens used in the different tests. Table 1

Surface Treatments

The passivation treatments studied in this work were immersion in commercial solutions of nitric acid, citric acid and FinishOne, while sulphuric acid and air exposure were regarded as reference treatments. Concentrations of the different solutions are given in table 2. Avesta Welding delivered the commercial nitric acid, citric acid and the FinishOne solutions. These solutions contain small amounts of other components, which are not specified in this report. The sulphuric acid was prepared from pure analytical grade and distilled water.

The specimens were immersed in the solutions at room temperature for 30 minutes after thorough cleaning. After passivation the specimens were rinsed in distilled water, and left in air for a minimum of 18 hours before any test was performed.

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Corrosion Tests

Pitting corrosion Measurements of the critical pitting temperature, CPT, were used to determine the treated material’s susceptibility to pitting corrosion. The critical temperatures were determined in 1M NaCl-solution at a constant potential, + 700 mV(SCE), according to ASTM G 150. This potentiostatic test procedure involves the use of a continuous temperature scan, which is performed in an electrochemical cell designed to eliminate the occurrence of crevice corrosion, i.e. the Avesta Cell [3]. The current is monitored and CPT is defined as the temperature at which the current density exceeds 100 µA/cm2 for more than 60 seconds.

Crevice corrosionThe critical crevice temperatures, CCT, were determined in 5% FeCl3 + 1% NaNO3 solution according to ASTM G 48 method F, with the discrepancy that the test solution was not acidified, the torque was 0.28 Nm, and the addition of nitrate. The nitrate was added in order to raise the critical temperature. Without nitrate, this grade has a CCT-value below the freezing point.

Electrochemical experimentsIn order to study the time effect of the passivation treatment, potential-time curves were recorded under open circuit conditions. The electrochemical measurements were performed in 0.5% deaerated sulphuric acid at room temperature. Potentials were measured against a saturated calomel electrode, SCE, which all potential-values in the text are referred to.

Surface Analysis

X-ray photoelectron spectroscopy (XPS) was used for surface analysis. The XPS analyses were performed with a PHI 5500 spectrometer using the monochromised AlK∞X-ray source (1486.6 eV) and a pass energy of 23.5 eV. The binding energy, BE, scale of the instrument was calibrated on Au and Cu: BE (Au 4f7/2) = 84.0 eV, BE (Cu 2p3/2) = 932.7 eV. A survey spectrum was first recorded to identify the elements present on the surface, then high-resolution spectra of Cr 2p and Fe 2p were recorded. For quantitative analysis the obtained spectra were fitted with different components, after back ground subtraction. The measured XPS intensities allowed evaluation of the composition and the thickness of the passive film. Detailed quantification is given elsewhere [4].

Duration Studies

In order to study the durability of the passivation treatments, i.e. if the passivation effect last even after some time in service, the passivated specimens were immersed in 0.5 weight-% sulphuric acid for 3 days. Dilute sulphuric acid was chosen as it is a fair common environment for stainless steels and also an environment where the investigated grade is resistant, i.e. showing a passive behaviour. During the 3 days, the open circuit potential was recorded. After this exposure the CPT measurements and the surface analysis were repeated.

Surface treatments investigated and cor responding concentration. Table 2

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Results Corrosion Tests

Critical Pitting and Crevice TemperaturesFigure 1 shows the obtained critical pitting temperatures after the different passivation treatments. The critical temperatures were determined both directly after passivation and after 3 days exposure in dilute sulphuric acid.

Treatment with nitric acid gives initially the highest CPT-value in comparison to the other treatments, which are almost equally good. After exposure in dilute sulphuric acid for 3 days the CPT-values have increased for all treatments and are now higher than for the freshly passivated specimens. The spread in data is rather high why no significant difference can be seen between the effects of the different passivation treatments. Determination of CCT gave the same temperature, 27.5°C, independent of surface treatment.

Electrochemical experimentsFigure 2 and 3 show the influence of surface treatment on the open circuit potential of the steel immersed in dilute sulphuric acid with time. Although large differences exist between potential – time behaviour during the initial period of exposure in dilute sulphuric acid (fig. 2) they arrive to the same value of around 0 mV(SCE) within 24 hours (fig. 3). Thereafter the potential increases very slowly reaching an equilibrium value of 50 to 75 mV at the end of the test.

Fig. 1 Critical pitting temperatures of surface treated 1.4404 according to ASTM G 150 measured directly after passivation and after exposure in dilute sulphuric acid for 3 days. The error bars give the standard deviation.

Fig. 2 Open circuit potential versus time recorded for 1.4404 during the initial period of exposure in 0.5 % H2SO4.

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Fig. 3 Open circuit potentials versus time recorded for 1.4404 during the whole test period.

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Surface analysis

Figure 4 shows an XPS survey scan of 1.4404 recorded after immersion in FinishOne solution at room temperature for 30 minutes. In the spectrum photoelectron and Auger electron signals from Ni, Fe, Cr, Mo, O and C are detected.

Fig. 4 XPS survey scan recorded from 1.4404 treated with FinishOne.

Fig. 5 High-resolution XPS spectra of Fe 2p and Cr 2p recorded from 1.4404 treated with solution 601 (nitric acid).

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XPS spectra recorded from narrow energy regions (Cr 2p3/2 and Fe 2p3/2) are shown in figure 5. It appears that both the oxide and the metallic states could be detected, indicating a thin passive film covering the metal phase.

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Figure 7 shows the chromium concentration in the passive films formed after passivation and after the following exposure to dilute sulphuric acid for 3 days. The figure shows that there is a big difference in chromium content directly after passivation. Passivation with nitric acid gives the highest chromium content while passivation with FinishOne gives the lowest chromium content. After 3 days exposure in sulphuric acid the chromium content has increased and is in the same order for all surface treatments, 52 to 60 atomic-%.

Fig. 7 Chromium content of the passive film after different surface treatments of 1.4404 measured directly after passivation and after exposure in dilute sulphuric acid for 3 days.

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DiscussionAll results show the same thing; – the passive film adjusts itself after the environment and reaches an equilibrium state with optimum properties after approximately one day.

The corrosion tests that have been performed directly after passivation show a slight improvement of the corrosion resistance after treatment with nitric acid, probably due to the formation of a thin chromium rich passive film. Passivation by nitric acid thereby decreases the time to reach equilibrium and the positive effect is obtained faster than for the other treatments. However, after the duration test, the properties of all treated surfaces are similar and in fact even better than directly after passivation.

Earlier investigations [4, 5] have shown a linear relationship between potential and passive film thickness. It has also been shown that the passive film becomes richer in iron at higher potentials. This explains why the passive layer on the surface, treated with FinishOne is thicker and also richer in iron than what the other treatments give. Since the obtained open circuit potential of the FinishOne-treated surface is high, one could

Fig. 6 The thickness of the passive film of surface treated 1.4404 measured directly after passivation and after exposure in dilute sulphuric acid for 3 days.

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Figure 6 shows the thickness of the passive films formed on the surface after passivation in the different solutions, both directly after the passivation treatment and after 3 days exposure in dilute sulphuric acid (duration test). Directly after passivation, the thickness of the passive films was different depending on surface treatment; Passivation with nitric acid gave the thinnest passive film, 1.8 nm, while passivation with FinishOne gave the thickest film, 2.7 nm. On the other hand, after the duration test, the passive film thicknesses approach the same value of about 1.7 to 1.8 nm independent of surface treatment.

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suspect an increased risk of pitting/crevice corrosion if the stainless steel will be used later in a chloride-containing environment. However, the high oxidation power caused by FinishOne is lost very shortly, within minutes as can be seen in figure 2. Pitting or crevice corrosion will hardly initiate during that period.

Instead the high potential has a beneficial side effect on the corrosion resistance. The high oxidizing power facilitate the removal of undesired iron contaminants which otherwise could form rust or act as initiation sites for corrosion. Metal iron and ferrous ions are oxidized further to ferric ions. When Fe3+ is the dominating ion it passivates the steel, which protects the surface not only from corrosion but also adhesion [6].

In this study the use of chelat agents has not been tested. However, investigations have earlier been performed by Balmer and Larger [7] to determine if the citric/chelat formulations were as good or better than the traditional nitric acid treatment. Their study showed that the chelat procedure gave significantly greater chromium enrichment at the surface and proved equal or better than the nitric acid passivation, but all their results were obtained directly after passivation.

If the time to reach optimum corrosion properties is of high importance, whereas the environmental and personal risks are of little consequence, this study has shown that nitric acid is the best choice. On the other hand, by using FinishOne, hazardous waste is decreased to a minimum. Hydrogen peroxide, which is the main component of FinishOne, is after use simply decomposed to pure oxygen and water. No extra rinsing after treatment is needed and no acidic or toxic waste is formed.

The result in this study shows that passivation is rarely needed for improved corrosion resistance. The optimum corrosion properties will be reached anyhow within one or two days. On the other hand, passivation is an effective way to clean the stainless steel surface from contaminants such as shop dirt or iron particles from fabrication operations. If the main reason is to clean the surface there is no point to use environmental unfriendly chemicals when better solutions are available for that purpose.

Conclusions

• The passive film adjusts itself after the environment and reaches an equilibrium state with optimum properties after approximately one day.

• Passivation with nitric acid decreases the time to equilibrium and the positive effect is obtained faster than for the other treatments.

• Duration studies show that all treatments have the same effect, after maximum 3 days, when regarding corrosion resistance.

• Chemical passivation treatments are not essential as the passive film forms spontaneously in the presence of oxygen. When applied to stainless steels, the main function of a chemical passivation treatment is to clean the surface, by removing free iron and other surface contaminants.

• FinishOne has initially a very high oxidising power which facilitates the removal of undesired iron contaminants, which otherwise can cause corrosion indirectly.

• Passivation should preferably be called cleaning.

References1. J.S. Noh, N.J. Laycock, W. Gao, D.B. Wells, Effect of nitric acid passivation on

the pitting resistance of 316 stainless steel, Corr. Sci. 42 (2000) 2069 – 2084.

2. R.W. Evans and D.C. Coleman, Corrosion Products Found in Pharmaceutical/ Biotech Sanitary Water Systems, The Definitive Journal of High-Purity Water, Part-1, Vol 16, No 8 pp. 68, Oct 1999,

3. Qvarfort R. New Electrochemical Cell for Pitting Corrosion Testing, Corrosion Science, Vol. 28, No 2, pp. 135–140, 1988.

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4. Wegrelius L, Passivation of Austenitic Stainless Steel, Doctor’s thesis, ISBN 91-7197-182-3, Department of Engineering Metals, Chalmers University of Technology, Gothenburg, Sweden (1995).

5. Brox B and Olefjord I, Preferential dissolution of iron during polarization of stainless steels in acids, Proc. on Stainless Steel -84, Göteborg, Sweden, The institute of Metals, London, 134, 1985

6. Holm B and Symniotis E, Formation and removal of undesired deposits on stainless steel surfaces during pickling, SIMR Research Report, IM-2000-553.

7. K.B. Balmer and M. Larter, Chelants Prove Practical for Cleaning and Passivation of Stainless Steel Parts, Precision Cleaning, Nov/Dec. p. 27–29, 1994.

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