Technical Bulletin provided by

Technical Bulletin provided by Copyright by Henkel 2003. All rights reserved. Data shown is typical, and should not be construed as limiting or necessarily suitable for design. Actual data may vary from those shown herein. FOR AUSTENITIC STAINLESS STEEL ALLOYS Essay No 45 / Rev. 00 Georg Henkel, MSE, PhD Benedikt Henkel, MSE The component s value is assured by its surface www.henkel-epol.com

/ 12 1. Introduction A characteristic of austenitic stainless-steel alloys, such as 1.4404, 1.4571, 1.4435, 1.4539, etc. in accordance with DIN 17440, is that they form a largely corrosion resistant protective layer on the surface. This layer which offers protection against chemical attack has the character of a passivation layer and allows the use of components in areas of application where other metallic materials are subject to corrosive attack, which results in failure or causes media to be contaminated accordingly by corrosion products. In this context, basically the question arises as to the structure and formation mechanism of the passivation layer of stainless steels, the relevant influencing parameters, the metrological assessment method, the limits of the layer resistance and any consequences of the (local) destruction of the passivation layer. 2. Structure and formation mechanism of the passivation layer Whereas in metallurgical terms austenitic stainless steels 1.4404/1.4435, 1.4571, 1.4539 as alloys are for the most part made up of the matrix component iron and the alloy components Cr, Ni, Mo, etc. are deemed to be alloying elements, analysis of the passive surface layer shows a completely different morphological structure : approx. 65 % Cr + Cr oxide and approx. 35 % Fe + Fe oxide. Only small amounts of Ni and Mo are found in the passivation layer, as can be seen from appropriate AUGER and ESCA analysis reports. Under normal conditions the thickness of the passivation layer lies at approx. 1.5 2.5 nm and shows a constant transition to the base material. In the protective layer, which contains a considerable amount of chromium oxide, with deposited Fe/Fe oxide, diffusion effects and the preferential affinities of Cr to O 2 has caused an appropriate phase inversion, which results in the passivation layer which can be clearly detected by metrological means. The preferred formation of chromium oxide or chromium hydroxide is due to the strong affinity of chromium to oxygen. The resulting chromium compounds are, in energetic terms, extremely stable with the result that reactions with other elements (e.g. corrosion reactions) are suppressed to a large extent. The term passivation layer is derived from this circumstance.

/ 12 The complete coverage of the surface by this layer rich in chromium oxide is a result of the fact that in addition to the given alloy percentage of approx. 18 % Cr due to local diffusion, an additional percentage of chromium atoms migrate to the surface and form chromium oxide/hydroxide. The fact of the presence of chromium due to the alloy and also undisturbed local post diffusion is much more pronounced in stainless steel surfaces which have been electrochemically polished than in mechanically processed surfaces, which is why electrochemically polished surfaces have considerably thicker and more stable or undisturbed chromium oxide layers with a correspondingly high Cr/Fe ratio and therefore have improved and calculable corrosion resistances. The complete coverage of the surface with chromium oxide/hydroxide in spite of the proportional minority compared with iron can be explained above all by the formation of oxide or hydroxide covering the surface. The Fe or oxide/hydroxide proportions are homogeneously deposited in the Cr/chromium oxide matrix. Figure 1: AUGER distribution in depth ESCA distribution of the elements of the chromium oxide rich passivation layer on the surface. a) mechanically polished b) electrochemically polished, removal 20 µm. The passivation layer has the typical characteristic of conducting electrons and also preventing the flow of ions, which usually stops corrosion circuits from being produced. 3. Influencing parameters of the chromium-oxide rich passivation layer The thickness, morphological structure or distribution of elements in the passivation layer (Cr/Fe ratio) is firstly influenced by the type of alloy with regard to the proportions of Cr, Ni and Mo. In addition, a dependency of the significant sizes of the passivation layer on the final surface treatment type can be recognized. The method of the passivation technology or surface treatment shows a considerable influence on the sizes of the passivation layer. In operation media and temperatures act on the passivation layer and may change it, with the result that repassivation phenomena can also be observed in this connection. You should take into account that the atoms and, in particular, molecules which form the passivation layer are in principle subject to a dynamic system of equilibrium.

/ 12 If environmental conditions (media/temperatures) considerably promote the disintegration of chromium oxides/hydroxides or the formation of Fe oxides/hydroxides, damage to the passivation layer and therefore a corrosion effect is unavoidable. Under normal atmospheric conditions without contamination with corrosive media, independent of the final surface treatment, passivation layer data stands at approximately Cr/Fe = 1... 2 and a layer thickness s = 1... 3 nm. The layer is basically transparent and shows the typical metallic appearance of steel. Highly oxidizing effects for instance anodically supported now allow the creation of a significantly thicker layer, whereas a procedure patented by INCO also allows the stabilization and hardening of this layer. In this case the phenomenon occurs where with increasing thickness of the passivation layer typical color effects are detected on the stainless steel surface, which are caused by light refraction in the passivation layer. This fact shows the principle of the coloring of stainless steel without the use of colorants and is an indication that the surface layer rich in chromium oxide has a typically amorphous structure. Figure 2: Cr/Fe ratio and development of the layer thickness depending on the type of alloy, surface treatment and media conditions 4. Description of the metrological assessment method Due to the submicroscopic conditions in the passivation layer, the measurement and metrological assessment relies on extremely complicated methods in physical terms, where the preparation of samples also exerts a considerable influence on the measurement results and their interpretation. Over the years, the Auger analysis and ESCA/XPS analysis have proved to be reliable measuring procedures for evaluating the chromium oxide rich passivation layer. The Auger analysis method allows the relatively exact analysis of the depth profiles of the individual alloying elements, where, in particular, the recording of the O 2 depth profile is of importance. Half the height of the oxygen peak is defined as the measure for the relevant thickness of the chromium oxide layer. The ESCA/XPS analysis method is used to record the alloying elements found on the surface and through the quantitative recording of these values allows a reproducible statement about the significant Cr/Fe ratio, where, with regard to Cr and Fe, metallic, oxidic and hydroxicidic proportions are totaled. For the future, we are assuming that other, with regard to the ESCA analysis method, more further developed methods will be used which will allow the measurement of any components removed from the surface during the measurement procedure and which will therefore allow more accurate statements to be made.

/ 12 5. Limits of the resistance of the passivation layer and the use of corrosion effects All known forms of corrosion of austenitic stainless steels such as.) pitting.) stress corrosion cracking.) crevice corrosion.) intercrystalline corrosion.) transcrystalline corrosion.) general corrosion.) contact corrosion, etc. are based on the fact that the once existing passivation layer was destroyed either locally or over a large area and as a result corrosive destruction of material can spread. The corrosive effect to be used depends on a series of factors which in addition to the effective media and effective conditions (e.g. temperatures) also have to taken into consideration the surface conditions with regard to topography, morphology and energy level. Classic forms of corrosive damage or destruction are.) pitting due to chromium loading.) local or surface damage due to stress corrosion cracking as a result of the mechanical operating conditions supporting the corrosive medium.) surfacing rouging effect with hot and ultra-pure waters and vapors.) surface corrosion effect due to applied contact corrosion coatings or in the area of untreated welding seams with reinforced Fe oxide contaminations or with the use of highly reducing media. In all the areas named, system-typical mechanisms can be detected which necessarily result in the (local) destruction of the thin chromium oxide rich passivation. Although the stainless steel manufacturers generally offer detailed resistance tables for the different alloy types, these recommendations are only helpful to a certain extent as they are based on the assumption that sample conditions are ideal and machining loads, surface treatments etc. are not taken into consideration. With this in mind it is highly recommended to call in expert opinions before selecting a material for a concrete application in order to come to an optimum decision from a technical and economic point of view.

/ 12 6. Methods for the controlled passivation layer formation (passivation) and assessment of the parameters In principle the formation of the passivation layer on the austenitic stainless steel surface corresponds to an oxidation of the surface and is therefore not only dependant on the condition/the preparedness of the surface but also on the oxidation conditions (medium, concentration, temperature and time). It is generally to be noted that from the chemical point of view each passivation layer represents a dynamic equilibrium building itself up and decomposing permantly which means in sum that there are stable conditions. At the same time the level of the chemical equilibrium depends on the surrounding conditions (temperature, medium etc.). With this in mind and with higher temperatures and higher proportions of oxigen a thicker chromiumoxide rich passivation layer can of course be formed, which, however, (without respective stabilization) under atmospheric conditions again decomposes to a reduced thickness level where it then remains thermodynamically stable. The influence of the surface condition in the sense of a passivation readiness has already been described above. It can easily be seen that electrochemically pretreated pure material structures can be passivated (oxidized) more effectively than mechanically deformed and structurally changed structure morphologies. This also shows clearly in the quality of the passivation layer when test measuring by Auger/ESCA. The oxidation conditions are of far less importance for the passivation result than the surface condition of the stainless steel surface. Unfortunately, this is not a widespread realization yet, particularily, as only since a few years by an optimized ESCA/AUGER-analysis simple metrological proofs for this realization in serialtests have been given. For the standard operations the formation of the chromium rich passivation layer in view of the quality of the layer (thickness, morphology) is independent of the kind of oxidation - in pure air - with pure water - with HNO 3 in concentrations of 5-30 % and metrologically tested in principle nearly the same, as long as the same pre-surfaces have been used. However, significant oxidation (passivation) periods have been noted in order to achieve the relevant layer eqilibrium - in air approx. 48-96 h - in water approx. 6-15 h - in HNO 3 in different concentrations approx. 30-120 min. During oxidation (passivation) it is nessecary to keep the stainless steel surfaces as clean as possible which means that clean conditions (air, water) have to be provided.

/ 12 For technical reasons (duration) passivations with pure air and water (deionate) are rather rare. From the practical point of view HNO 3 -passivations play a more imortant part. Regarding the passivation layer morphology it needs to be added that layers produced in acid solution not only show chromium oxides but also chromium hydroxides, whereas these coatings were not noted after dry passivation. From technical resp. corrosion technical point of view however this is not relevant. In this connection the following parameters for the passivation process are of importance : - Kind of medium, concentration - Temperature - Time and perhaps the adding of chelat. HNO 3 as medium is usually recommended as being most reasonable in price and chemically oxidizing best whereby it must be observed that HNO 3 irrelevant of its concentration does not have a cleaning but only an oxidizing (passivation formation) effect. This has in the past not been described as clearly. Contaminated surfaces (not ready for passivation) have to be cleaned chemically resp. pickled by means of other procedures beforehand. For the passivation process the addition of chelat is not of importance for the chemical oxidation process, however, it allows a parallel removal of foreign metals by complex formation (cleaning effect) and insofar helps with marginally contaminated surfaces (i.e. Fecontamination). Precleaned stainless steel surfaces (chemically pickled / electropolished) are not measurably improved by adding chelat to the passivation solution. Mechanically pretreated surfaces, however, are improved measurably as chelat has a slight pickling effect. The concentration of the HNO 3 -passivation solution of 5-30 % (independent of the questions of disposal) in connection with temperature (20-35 C) and time (10-120 min.) during the passivation process has no influence on the quality of the passivation layer (measurable by ESCA/Auger) of electropolished and therefore best conditioned stainless steel surfaces and only little influence on so called mb-surfaces. Principally, and due to morphological reasons only significantly weaker passviation layers are formed on the mb-surfaces which also explains the respectively reduced corrosion resistance. These facts also explain the known noticeably increased rouging effects on mbsurfaces and, furthermore, in connection with much more unfavourable topographical and morphological surface conditions of mb-qualitites, respective significantly more difficult derouging effects in comparison to electropolished surface conditions. Serial tests of 1.4404 (316L) regarding this problem and passivation layer evaluations (layer thickness, Cr/Fe-Ratio) analysed by ESCA/Auger have lead to the results described in figure 3.

/ 12 The interpretation of the results show that the concentration of the HNO 3 -solution does not have a significant influence on the maximum possible formation of a passivation layer on this surface condition. This means that it can securely be operated with 5 % HNO 3 at temperatures of 20-25 C over 60 min. in order to achieve reduced, thus less resistant but stabel layer conditions. Earlier recommendations in the ASTM A 380 with clearly increased HNO 3 concentrations are obviously based on the hopes, particularily as at the time of the establishment of standards the objective testing measurements through ESCA/Auger were not yet totally accessible and for different mb-qualities with typical surface contaminations quite varying passivation layer results had to be expected. The studies at hand, however, show that even with a higher HNO 3 concentration a measurable improvement of the passivation layer can not be achieved as long as the parameters temperature and time are being used optimally. Parallel cleaning measures during the passivation process are much less due to a higher HNO 3 concentration while the added chelats, tensides and other cleaning substances in the passivation solution are much more effective for an optimal final cleaning. In this connection especially for the necessary cleaning process after the installation and before the passivation the expected kinds and extent of contamination and the respective cleaning parameters need to be coordinated and recorded. The stability of the passivation layer results regarding the parameters concentration, temperature and time are explained by the fact that electropolished stainless steel surfaces are ideal for the passivation- and oxidation process due to their pure austenitic matrix structure and the fact that no distrubing contaminations as with mb-surfaces causes difficulties. Insofar these results are of importance for repassivation processes after CIPoperations as well as for perhaps necessary derouging operations. On the basis of a number of studies this abstract is in accordance with per today unpublished results of our colleagues in the USA (Report Interpharm New York 2000), the objective morphological conditions of which can reliably be proven by Auger/ESCA-analysis resp. explain the majority of practical behaviour phenomena of mb-surfaces as well as of electropolished stainless steel surfaces. The resulting practical passivation recommentation for standardized proceeding regulations for alloys of 1.4404 / 4435 is : Electropolished surfaces: HNO 3 concentration 5-10 % (Cl - < 10ppm) T = 20-35 C t = 30-45 min. whereby longer time periods neither have a positive effect on the material nor a negative.

/ 12 Mb-surfaces : HNO 3 concentration 5-10 % (Cl - < 10ppm) T = 20-35 C t = 90-120 min. whereby longer time periods neither have a positive effect on the material nor a negative. Hereby it is to be noted that the passivation layer (when firstly- and repassivated) which is formed on electropolished surfaces is of a significantly higher quality regarding layer thickness and formation than that on mb-surfaces. For the successful operation practically tested mixtures of chemicals such as HENKEL HC1100 for passivations, HENKEL HC500 (Basis NaOH) for cleaning/rinsing and HENKEL HC1106 for derouging are recommended. The Ferroxyltest according to ASTM A380 enables the local testing of a successful passivation by way of simple chemical analysis. 7. Summary Description of the build-up mechanisms of the passivation layer in austenitic stainless steel alloys, their influencing parameters and measurement techniques for assessment and testing methods. In addition, the consequences of a (local) passivation layer destruction and the relevant corrosive resultant effects are clearly described. Please contact us for further information

/ 12 Fig. 1a): mechanically polished K 240, passivated.) AUGER ANALYSIS Atom % Passivation layer Transition layer Basic material = Fe Cr O 2 = S Thickness of passivation layer: s = 1.1 nm Depth (nm).) ESCA result: Cr/Fe = 1.01.) Crystalline structure analysis Georg Henkel, MSE, PhD

/ 12 Fig. 1 b): electrochemically polished, removal 20 µm, passivated.) AUGER ANALYSIS Atom % Passivation layer Transition layer Basic material = Fe = Cr O 2 S Depth (nm) Thickness of passivation layer: s = 2.6 nm.) ESCA result: Cr/Fe = 1.72.) Crystalline structure analysis Georg Henkel, MSE, PhD

/ 12 Figure 2 1.0 1.5 2.0 2.5 Cr/Fe Type of alloy 1.4404/1.4435 1.4539 Final surface treatment mechanically polished electropolished Passivation technology HNO 3 HNO 3 + chelate Operating temperature high temperature low temperature Media effect reducing oxidizing Thicknes s of passivati on layer (nm) 3.0 2.0 1.0 Type of alloy 1.4404/1.4435 1.4539 Final surface treatment mechanically electrochemically polished polished Passivation technology HNO 3 HNO 3 + chelate Operating temperature high temperature low temperature Media effect reducing oxidizing Georg Henkel, MSE, PhD

/ 12 Figure 3 : Mb-surfaces 1.4404/4435 (cold rolled or ground K240 or cold drawn) Producible layer thickness 0,8 1,1 nm Cr/Fe Ratio : 0,85 1,01 Parameter for obtaining resp. stable chromiumoxide layer conditions HNO 3 -concentration 5 %: t = 60 min t = 45 min t = 40 min HNO 3 -concentration 10 %: t = 60 min t = 40 min t = 30 min HNO 3 -concentration 20 %: t = 60 min t = 40 min t = 30 min HNO 3 -concentration 30 %: t = 60 min t = 40 min t = 30 min Figure 4 : Electrochemically polished 1.4404/4435 (electropolished removal approx. 20 µm) Producible layer thickness 2,4 2,6 nm Cr/Fe Ratio : 1,56 1,72 Parameter for obtaining resp. stable chromiumoxide layer conditions HNO 3 -concentration 5 %: HNO 3 -concentration 10 %: HNO 3 -concentration 20 %: HNO 3 -concentration 30 %: Georg Henkel, MSE, PhD

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