It is important to ensure that all forms of interaction between the surface of a pipe and the medium flowing through it are prevented, particularly in the pharmaceutical industry and in biotechnical applications. In the search for a solution to this problem, more weight is often attached to cost considerations than to choosing the most suitable stainless-steel quality.
Dr. Georg Henkel
Under certain conditions, stainless-steel alloys form a thin passive layer of chromium oxide, which is what lends the surface its non-rusting character. This phenomenon was recently subjected to a closer examination against the background of increasing occurrences of corrosion-related damage. Auger and ESCA methods were used to analyse surface layers with a relatively homogeneous structure. EDX analyses and SEM scans were employed on the other hand for surfaces distinguished by inhomogeneities, such as local inclusions.
Protective chromium-oxide layers low in iron
Only electrochemically polished surfaces have the required morphological purity to permit homogeneous, stable, chromium-oxide passive layers to form with relatively few disturbances. This cannot be achieved if the surfaces are manufactured by mechanical means. The analyses were therefore conducted on test specimens made of 1.4435 stainless steel with an electrochemically polished surface, an anodic removal depth of 30 to 40 µm and a resulting total profile height of 0.2 to 0.3 µm. The tests yielded a typical morphological profile of the kind shown in figure 1. Although the base alloy normally has a chromium-to-iron ratio of 0.3:1, this ratio is 1.5:1 directly on the steel surface on account of the passivation processes that take place there. This is proof that a stable chromium-oxide layer has formed. The typical chromium-to-iron ratio of this layer decreases continuously as the measuring depth increases, and finally reaches the base alloy value of 0.3:1 again at around 5 to 6 nm. This constitutes an inversion phenomenon with a transition from a chromium-dominated phase zone on the surface to an iron-dominated phase in the centre. The transition is continuous. The chromium oxide-dominated matrix layer, which helps to prevent corrosion, has a typical thickness of at least 2 to 5 nm (safetron ep). This protective layer is poor in iron and thus also in iron oxide; it contains nickel, nickel oxide, molybdenum and molybdenum oxide as well as water molecules that are intercalated by bridge bonding (Fig. 2). It is likewise important for corrosion inhibition that ion transport is effectively prevented. Compared with passive layers that are less well-developed (chromium-oxide layer 2 nm), the resistance of the surfaces described here is far superior.
Owing to the dynamic nature of this kind of passive layer, it is essential to collate all the available information, as well as basic conditions and constraints, relating to composi-tion, decomposition, decay and possibly recomposition. This complete list of corro-sion-causing parameters, combined with know-how about the mechanisms taking place, is extremely useful for drawing up maintenance schedules detailing the time framework for essential cleaning, inspection and passivation work.
Stability of stainless-steel surfaces
The findings described below refer above all to the 316L alloy types, which are currently among the most widely used (1.4404 and 1.4435).
In day-to-day operation, doubts often persist about the stability and quality of stainless-steel surfaces as well as the possible finan-cial consequences. These uncertainties are moreover aggravated by instances of corrosion-related damage for which there is no immediate explanation. Irrespective of the strength aspect, stainless-steel surfaces should resist corrosion in the area in contact with the medium, exhibit a neutral or inert behaviour with respect to this medium and facilitate residue-free CIP cleaning.
In plants designed for the pharmaceutical industry, the majority of stainless-steel sur-faces in the area in contact with the medium are electropolished. The analyses therefore focussed on these surfaces. All media which have no adverse effect on the corrosion resistance of the boundary stainless-steel material – 1.4435 for example – are also characterised by satisfactory, or at least adequate, behaviour when it comes to residue-free cleaning and neutrality. These conclusions were backed up by tests conducted to determine the change in the bacteria count, for instance by determining the pyrogenity of the test substances.
When resistance to corrosion from a range of media is assessed, electropolished, stainless-steel surfaces are invariably superior to mechanically treated surfaces. This is due to their considerably more compact, more uniform, chromium-oxide passive layer. Electropolished surfaces enable the theoretical corrosion resistance of the particular alloy to be utilised practically without restriction. This ensures above average design and planning security.
The conducted tests yielded the following limits of operation for the 1.4435 material, depending on the actual medium which is used:
• Solutions containing chloride, whereby the concentration, temperature, exposure time and state of motion are key parameters,
• desalinated water, in which case the conductivity, temperature, oxygen content, etc. are crucial,
• superpure steam,
• reducing alkalis, the effectiveness of which is likewise dependent on their concentration, temperature and exposure time.
If the 1.4435 and 1.4404 materials are exposed to the above-mentioned media, it is not uncommon for defects to be observed that may be production or assembly-related. These critical applications are however extremes, where even minute processing or treatment errors can lead to pitting, intercrystalline corrosion, crevice corrosion, etc. The nature and extent of these corrosion symptoms are highly diverse and are often the result of combined actions.
Empirical experience is confirmed by laboratory tests, which have shown welds and their immediate environment to be significantly more prone to corrosion than the surrounding base material. Corrosion processes frequently begin at welds and in the areas in the direct vicinity of them. They subsequently set other self-catalysing processes in motion.
It is generally true to say that if stainless-steel components come into contact with the media mentioned above, they need to be planned and manufactured particularly carefully. Moreover, in-process surface tests and inspections are essential. The 1.4539 stainless-steel quality (safetron) offers added safety for especially critical applications.
Bruno Dockweiler
Fax: ++49/40/713 04166
Further information cpp-248
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