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Determining optimal conditions for effective passivation requires a good understanding of corrosion, derouging, and passivation itself. How can one monitor these distinct processes? What are the techniques that can be used and what can one learn by monitoring these processes under controlled conditions? This paper discusses various processes, monitoring techniques, and the use of these tools for derouging and passivation of product contact surfaces.
Introduction
In the pharmaceutical industry product contact surfaces are typically fabricated from 300 series of stainless steel, a material chosen for its corrosion resistance. Surfaces are typically passivated upon installation, but with time reddish brown rouge is observed over steel surfaces, especially in water systems. Although rouge and passivation differ, from the corrosion science perspective they have some common elements. To gain a better understanding of these two processes it is important to understand the properties of metallic substrate in general with respect to exposure to certain environments or the corrosion process.
All metallurgies, upon exposure to a given environment, will undergo degradation or corrosion. However, the nature of corrosion and the rate of corrosion depend upon the type of metallurgy and the nature of the aggressive environment to which it is exposed.
Corrosion
The first step in corrosion is formation of a thin liquid film on the surface exposed to the atmosphere or bulk liquid phase. The dissolved aggressive species in the liquid react with the metal via a redox reaction, resulting in the oxidation of the metal and the reduction of the aggressive species. As it relates to rouge, the base metallurgy of stainless steel is ferrous (Fe) with some chromium (Cr), molybdenum (Mo) and trace levels of other components. The overall corrosion reaction can be represented by two half reactions as follows:
Thus the base metal such as Fe or the minor components will undergo oxidation resulting in the liberation of dissolved ions (e.g. Fe+2) at the reactive interface of the metal. This is referred to as the anodic (oxidation) half reaction. The exposure of the metal to alkaline water exposed to air will result in the reduction of the aggressive species, such as dissolved oxygen, to generate hydroxide ions (OH-). This is referred to as the cathodic (reduction) half reaction. The dissolved metal ions very close to the reactive interface will now react with the hydroxide ions to precipitate as iron hydroxide. The final corrosion reaction product deposited on the metal surface is generally a mixture of metal hydroxides and various oxides.
In acidic water the oxidation half reaction remains the same and the reduction half reaction will result in the liberation of hydrogen gas. One can extend this representation to other reactive chemical species that may contact the metal surface during manufacturing. The net result is dissolution of the metal and the formation of the dissolved metal ions or precipitated reaction product on the metal surface. It is important to note that not all metallurgies are readily susceptible to corrosion. In a given environment, low chromium stainless steels will undergo corrosion at a higher corrosion rate than high chromium based stainless steels.
Rouge
In the pharmaceutical and the biotechnology industries the term "rouge" refers to the reddish brown deposit observed on pipes and tanks. Depending upon the vessels' age, this corrosion product is relatively loosely adhering to the surface. This reddish brown deposit is a corrosion product resulting from the corrosion reaction at various sites in the system. The corrosion product then deposits on these sites or distributed in the entire system with the liquid flow and ultimately deposited on the metal surface. The chemical identity of this corrosion product is predominantly iron oxide and hydroxide.
For corrosion depends on the right conditions. These include, but are not limited to, the type of metallurgy, the pH of the liquid in contact with the metal, identity of the dissolved aggressive chemical species, liquid temperature, and others. In order to keep these surfaces clean and to maintain good water and product quality, it is necessary to derouge these surfaces periodically. Derouging is the process of removal of the rouge or the metal oxides and hydroxides.
Passivation
Passivation of stainless steel, a corrosion reaction carried out under controlled conditions, grows a very thin, uniform, adherent corrosion product film that is protective against further corrosion. The chemical nature of this film is primarily mixed metal oxides and hydroxides. Although the exact chemical nature of the film is unknown, the literature indicates that it is composed of mixed metal oxides including iron and chromium. Depending upon the base metallurgy, there may be traces of oxides and hydroxides of other metals. However, the major contribution to the protective nature is a consequence of the enrichment of the Cr content in this film compared to the Cr content of the bulk metal.
Techniques for Monitoring Passivation and Derouging
For effective derouging it is critical to remove any organic residue before the derouging step. As pointed out earner, derouging is the removal of metal oxides. The progress of the derouging can be monitored by monitoring the concentration of dissolved Fe in the derouging solution. As iron oxide dissolves, the concentration of dissolved Fe increases initially in the derouging solution, and eventually as the derouging process ceases the Fe concentration levels off. A relatively constant value of dissolved Fe with time is an indication of the conclusion of derouging process. Monitoring the concentration of dissolved Fe can be accomplished with a broad range of analytical techniques, however, the most convenient techniques in the derouging solution are spectrophotometry or atomic absorption.
De-greasing is the first step in passivating metal surfaces effectively. This can be accomplished by treating the surface with an alkaline detergent cleaner to remove any organic oily or fatty residue from the surface. De-greasing is then followed by removing the inorganic nonprotective oxide deposit (derouging). After the surface is free of the oily residue and the inorganic oxides it is ready for passivation.
How can one monitor all of these processes? There are two aspects to this: monitoring the removal of the oxide deposit and measuring the corrosion rate (or the corrosion current) as the passivation process progresses. Finally the protective passive film is characterized by surface sensitive techniques. Since corrosion and passivation are redox reactions that occur at different stages of the surface treatment of the product contact surfaces, both of these processes can be monitored by electrochemical means. The electrochemical techniques indicate the tendency for corrosion and passivation for a given metallurgy when exposed to a given environment or solution at a given temperature.
Let us consider the corrosion rate (how fast the corrosion reaction is progressing) monitored by an electrochemical means. The corrosion rate is typically expressed as mpy (mile per year), and it directly proportional to the current (also referred to as the corrosion current) monitored.
Corrosion Rate = K DI / DE
Where K is a constant, I represents the current in amperes as the corrosion reaction proceeds, E is the applied potential in volts, and D represents a change. For a constant potential (E) the corrosion rate is then expressed as:
Corrosion Rate = K I
Thus for a constant potential, the corrosion rate is directly proportional to the current measured.
Several electrochemical techniques can be used to monitor the corrosion reaction. However, for our application the two most useful techniques are the linear polarization and potentiodynamic polarization. Linear polarization monitors the instantaneous general corrosion rate as it progresses, while potentiodynamic polarization measures both corrosion rate and the tendency for passivation and pitting (or localized corrosion).
In light of our interest in the passivation process, we will focus on the potentiodynamic technique only. This technique involves scanning (at a given scan rate) the applied potential with respect to the equilibrium potential either cathodically or anodically, and the measuring the resulting current logarithmically. When the stainless steel metallurgy is exposed to a given solution, at equilibrium or at rest, the potential of the working electrode (electrode of interest) with respect to a reference electrode, is the equilibrium potential for the system and it is referred to as Ecorr. As the potential is scanned greater than Ecorr (in the anodic direction) the conditions favor corrosion . For 300 series stainless steel the current continues to increase to a certain level and then drops. This indicates the transition from active to a passive region, or from the corrosion step to the formation of the passive film.
As the passive film forms the current drops, until it reaches a lowest possible value for the given environment and remains at that level This region is referred to as the passive region. The reason the current does not rise is that the protective passive film protects the metal surface against further corrosion, despite the fact that the potential is increased beyond the equilibrium potential. The increased potential typically means that excess energy is being applied to the surface to facilitate corrosion. As the potential is scanned to higher voltages, the current will remain relatively unchanged until it reaches a value. At this point the passive film loses its protective properties under the applied potential, and it undergoes pitting or localized damage and therefore corrosion. This potential is called the pitting potential (Ep).
The region beyond the pitting potential is called the transpassive region. Lower-grade carbon steel exhibits a continuous rise in the current beyond Ecorr as the potential is increased. This behavior indicates a continuous increase in corrosion rate with the formation of no protective passive film, and therefore no transition from the active to the passive region.
The polarization curves in 34% nitric acid and the phosphoric acid based formulated product (CIP--200 at 4oz/gal dilution), both at 55 °C, after one minute equilibration before measuring the polarization curve are shown in Figure 1. Both chemistries exhibit typical passivation curves. During the one minute equilibration time in the respective solution before the polarization curves are measured, the electrodes are spontaneously passivated. Therefore the polarization curves do not exhibit the active to passive transition region. The optimum conditions for effective passivation will vary for different chemistries. It is therefore critical to determine the optimum conditions under controlled laboratory conditions before the field application.
Characterization Of The Passive Film
After a metal surface has been passivated it is desirable to characterize the protective passive film to confirm that the conditions utilized for passivation did indeed result in a passive film. Although numerous surface analysis techniques exist, two are useful for characterizing the passive film: Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). Both techniques are vacuum techniques that analyze the depth profile of the elemental composition of the passive film at atomic resolution.
As discussed earlier, the passive film is primarily a mixed metal oxide and hydroxide film; it can therefore be analyzed for the depth profile of the various elements that make up the passive film. These include ferrous, oxygen, chromium, and any other elements of interest. With these techniques the probe beam penetrates through the depth of the passive film and identifies various elements and their relative concentration. These techniques also determine the relative film thickness. Since the primary component responsible for imparting the protective properties of the passive film is Cr. it is desirable to determine the depth profile ratio of Cr to Fe. For the determination of the Cr depth profile, XPS is a better approach than AES.
Conclusions
It is important to determine the appropriate conditions for derouging and passivation under laboratory controlled conditions to ensure the success of the chemistry. It is not sufficient to determine the condition for derouging and passivation. The resulting passive film must also be characterized using appropriate surface sensitive techniques. Potentiodynamic polarization technique can monitor the passivation process as it progresses, thereby facilitating the optimal conditions for the process. At the same time, XPS can help determine the Cr enrichment in the passive film.