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ROUGING

A DISCOLORATION OF STAINLESS STEEL SURFACES

 

Rouging is a thin film, usually reddish-brown or golden in color, of iron oxide or hydroxide, typically on stainless steels. The contrast between this film and shiny metal accentuates this aesthetics problem. The rouge film typically wipes off easily with a light cloth (Figure 1), but it reforms while the process fluid is in contact with the stainless steel. This problem is most chronic in the pharmaceutical industry on the interior surfaces of high purity water (i.e., water for injection, WFI) distillation units, storage tanks, distribution systems (piping, valves, pump housings, fittings, etc.) and process vessels.

As stated, rouge is ferric oxide (i.e., rust), but the film may contain not only iron but also chromium and nickel compounds in various forms, and hence the film may vary in color and tenacity (Figure 2). Rouging is experienced more on Type 304/304L stainless steel than on Type 316/316L, and less on electropolished surfaces than mechanically polished surfaces. Particles of rust can become dislodged and be dispersed throughout a piping distribution system, often collecting on in-line filters.

Stainless steel is "stainless" owing to the fact that the alloy forms a thin, protective, tenacious, transparent oxide film that protects it against destructive corrosive species in aqueous solutions. This film is composed of chromium oxide, and is said to make the steel "passive" against corrosion. The exact nature of this film is a continuing subject of debate, and in fact its exact structure and nature may vary, depending on a number of variables. It is known that the film forms very rapidly in most environments that are not actively reducing. Exposure to moist air will provide this passivation within a matter of minutes, and the film will thicken with time. Exposure to oxidizing acids, particularly nitric acid, only speeds up that which will occur naturally. Therefore, the treatments that are described as "passivation" are in reality cleaning procedures since passivation of clean, uncontaminated stainless steel occurs spontaneously, and no further chemical treatment is needed to facilitate it.

The passive layer on the surface of stainless steels can breakdown by the interaction of ultra pure water, which is devoid of ionic species, leading to rouging, or rust blooms. The ionic pull of the water is strong enough to strip the protective chromium oxide off the steel surface. This results in the stainless steel having to re-passivate by reforming another layer of chromium oxide film, which incorporates the rouge causing discoloration. During the brief time it takes to repassivate, a thin layer of the stainless steel dissolves, or corrodes. The major elements composing stainless steel are iron, chromium and nickel. The chromium and nickel ions are soluble and go into the bulk solution. The iron, however, precipitates above a pH of 3 as iron hydroxide that readily oxidizes to ferric oxide, which is red in color (i.e., rouge). If this progresses uniformly across the surface of the steel, and the depassivation / repassivation process is cycled many times, then the surface of the stainless steel takes on a light golden to dark brown appearance depending on the ionic state of the various oxide layers and their depth (Figure 3).

Another process, which is more damaging, is the creation and propagation of pits. Non-metallic inclusions, such as sulfides, oxides, etc., are an inherent result of alloy production. They are dispersed throughout the metal and are highly susceptible to attack by aggressive environments. Typically these inclusions are dissolved in a particular solution or environment and leave a micro-void behind. This void becomes an occluded cell where solution chemistry can be different from the bulk solution. If the stainless steel does not readily re-passivate, then corrosion attack within the void can propagate. The corrosion products within the now formed pit spill out onto the bulk metal surface producing localized rouging or rust blooms (Figure 4). After a pit initiates, propagation of the pit may occur and progresses until a through-wall penetration occurs. Alternately, if the pit heals, the activity ceases and no propagation occurs. However, the red rouge product remains as a telltale indication that something has occurred.

Regarding pitting of stainless steels in chloride-containing environments, as are frequently encountered in pharmaceutical and chemical process industries, it has been found that for a particular stainless steel in a particular chloride environment, there is a specific temperature above which certain stainless steels begin to immediately corrode and below which corrosion does not begin in an indefinitely long time. This temperature is called the Critical Pitting Temperature (CPT). Although corrosion begins immediately when the stainless steel is exposed above the CPT, as measured by potentiostatic monitoring (ASTM G 150), it generally takes as much as 24 to 720 hours of exposure for pitting to develop to the extent that permits observation of pit sites, as typified by localized rouging, even at 20X magnification aided by surface scratching techniques. Therefore, there are many instances in cyclic service or batch production where stainless steels are briefly, perhaps for one to a few hours, exposed to conditions above the CPT. Although corrosion damage is accumulated, intense maintenance and cleaning practices may permit such equipment to be used for extended service life. However, anything that extends the period of exposure above the CPT, whether in larger equipment or an unrelated deviation in process schedule, can lead to severe corrosion damage during a single cycle of a batch operation.

 

Figure 1. Rouge wipes easily off the stainless steel substrate, but will reform Figure 2. Rouging comes in many colors and degrees of tenacity.
 
Figure 3. After repeated cycles, a clean stainless steel surface can take on a multi- colored appearance. Figure 4. Rouge or rust blooms identify pit sites.

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