Malcolm Nichol has been investigating the phenomenon of rouging to better understand its nature and causal factors, using sophisticated analytical techniques such as confocal laser microscopy and, in this article, shows images from a test programme.

Corrosion-related problems

The pharmaceutical and biotechnology industries in particular experience corrosion-related problems with their stainless steel process plant and equipment.

A common problem in the industry is a phenomenon called 'rouging' which, as the name suggests, is a rouge-coloured deposit that forms in high purity water and clean steam systems.

Chemical analysis has shown that rouge is a form of rust containing oxides of iron and other elements such as chromium, nickel and molybdenum.

Apart from the fact that the deposit is undesirable from an aesthetic viewpoint, its presence can in certain circumstances compromise the aseptic/hygienic integrity of the process system or, in the worst cases, impact product quality.

Merits of electropolishing

There has been much debate about the merits of electropolishing compared to mechanical polishing and its influence on the formation of rouging.

It is thought that the dissolution of the alloying elements from the bulk alloy is influenced in some way by the type of surface finish, which could be one of the key factors in the development of rouging.

Here at Enspec, we have been investigating the phenomenon of rouging to better understand its nature and causal factors, using sophisticated analytical techniques such as confocal laser microscopy. In this article, we show images from one of our test programmes.

A LEXT OLS3100 3D Confocal Laser Scanning Microscope was used to assess a series of corrosion 316L stainless steel test coupons which had been exposed to ultrapure water at 85°C.

The test programme had been developed to study the effects of surface finish and metallurgy on the formation of rouging on stainless steels in high purity waters.

Rouging is relatively common in high purity water processing systems (extremely low conductivity; <1.3 μS/cm at 25°C) used for the production of drugs and other parenteral products.

The greater depth of field at higher magnifications made it possible to acquire high-resolution, true colour images of rouge particulates and deposits, and other microtopographical features. 

Figure 1 shows clusters of rouge deposits on the rough ground surface of a stainless steel coupon that had been exposed to purified water at 85°C for 12 hours.

Figure 1: Clusters of rouge deposits on a stainless steel test coupon with a ground surface finish

The ability to manipulate true-colour three dimensional images to any viewing angle provided a greater understanding of the morphology and deposition characteristics of the deposits as can be seen in Figure 2.

Figure 2: LEXT 3D true colour image of rouge deposits which had accumulated in the grinding/polishing grooves

Summary

The results of the investigation showed that the rouge particles formed and accumulated at surface discontinuities, such as small elongated cavities which were created during the mechanical polishing procedure.

The rouging also appeared to develop at the sharp edges along the grooves, indicating the presence of a local anodic dissolution process of the metal at sharp geometric features.

Laser scanning microscopy is not commonly employed to investigate rouge-related problems, but it is a technique that offers the opportunity to gain a greater understanding of the relationship between oxide particle/film development and surface topography.

(Note: the physical size of the microscope is such that it is a lab-based piece of equipment that cannot be used in-situ in a plant/vessel etc. However, it can be used to examine deposits on sterile wipes or small specimens which exhibit rouging.) 

Author: Malcolm Nicol, Enspec Technology Ltd