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ouging is a thin film—usu-ally reddish-brown or gold in color—of iron ox-ide (Fe

2O

3 ) or hydroxide

[Fe(OH3 )], typically on

stainless steels (SS). Thecontrast between the film

S31603), and less on electropolishedsurfaces than mechanically polished

surfaces. Particles of rust can becomedislodged and be dispersed throughouta piping distribution system, often col-lecting on in-line filters.

SS is “stainless” because the alloy forms a thin, protective, tenacious,transparent oxide film that protects itagainst destructive corrosive species inaqueous solutions. This film is com-posed of chromium oxide, and is saidto make the steel “passive” against cor-rosion. The exact nature of this film

remains debatable. In fact, its exactstructure and nature may vary depend-ing on a number of variables.

It is known that the film forms very rapidly in most environments that arenot actively reducing. Exposure tomoist air provides this passivation

 within a matter of minutes, and the filmthickens with time. Exposure to oxi-dizing acids, particularly nitric acid(HNO3 ), only speeds up the process.Therefore, the treatments described as

“passivation” are actually cleaning pro-cedures—passivation of clean, uncon-taminated SS occurs spontaneously,and no chemical treatment is neededto facilitate it.

The passive layer on the surface of SS can be broken down by the interac-tion of ultrapure water, which is de-

 void of ionic species and leads to roug-ing or rust blooms. The water’s ionicpull is strong enough to strip the pro-tective chromium oxide off the steel

surface. This causes the SS to re-passivate by forming another layer of oxide film, which incorporates therouge and causes discoloration. Duringthe brief repassivation period, a thinlayer of the SS dissolves, or corrodes.

The major elements in SS are iron,chromium, and nickel. The chromiumand nickel ions are soluble and go intothe bulk solution. The iron, however,precipitates above a pH of 3 as Fe(OH)3

that readily oxidizes to Fe2O3, which is

red (rouge) in color. If this progressesuniformly across the surface of thesteel and the depassivation/repassiva-

PHORGOTTEN PHENOMENA 

Rouging— A Discoloration of 

Stainless SteelSurfacesR ICHARD A. CORBETT, Corrosion Testing Laboratories, Inc.

R and shiny metal accentuates this aes-thetics problem. The rouge film typi-cally wipes off easily with a light cloth (Figure 1), but it reforms while the pro-cess fluid is in contact with the SS. This

problem is most common in the phar-maceutical industry on the interior sur-faces of high-purity water (water for in-

 jection) distillation units, storage tanks,distribution systems (piping, valves,pump housings, fittings, etc.), and pro-cess vessels.

 As stated, rouge is Fe2O3 (rust), butthe film may contain not only iron butalso chromium and nickel compoundsin various forms, and hence the filmmay vary in color and tenacity (Figure

2). Rouging occurs more often on type304/304L SS (UNS S30400/S30403)than on type 316/316L (UNS S31600/ 

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tion process is cycled many times, thesurface of the SS takes on a light-goldto dark-brown appearance dependingon the ionic state of the various oxidelayers and their depth (Figure 3).

 Another process, which is moredamaging, is the creation and propa-gation of pits. Nonmetallic inclusions

such as sulfides and oxides inherently result from alloy production. They aredispersed throughout the metal andare highly susceptible to attack by ag-gressive environments. These inclu-sions typically dissolve in a particular 

solution or environment and leave be-hind a microvoid. This void becomesan occluded cell where solutionchemistry can differ from the bulk so-lution. If the SS does not readily repassivate, then corrosion attack 

 within the void can propagate. Thecorrosion products within the now-

formed pit spill out onto the bulk metal surface, producing localizedrouging or rust blooms (Figure 4). Af-ter a pit initiates, it may propagate andprogress until through-wall penetra-tion occurs. Alternatively, if the pit

FIGURE 1 FIGURE 2

Rouge wipes easily off the SS substrate but will reform. Rouging comes in many colors and degrees of tenacity.

FIGURE 3 FIGURE 4

 After repeated cycles, a clean SS surface can take on a multicoloredappearance. Rouge or rust blooms identify pit sites.

heals, the activity ceases and nopropagation occurs. However, the redproduct remains to indicate thatsomething has occurred.

SS pitting is known to occur in chlo-ride-containing environments such asthose frequently encountered in thepharmaceutical and chemical process

industries. It has been found that, for aparticular SS in a particular chlorideenvironment, there is a specific tem-perature above which certain SS beginto corrode immediately and below 

 which corrosion does not begin for an

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indefinite time. This temperature iscalled the critical pitting temperature

(CPT). Although corrosion begins im-mediately when the SS is exposedabove the CPT (as measured by pentiostatic monitoring [ASTMG1501]), it generally takes from 24 to720 h of exposure for pitting to de-

 velop to the extent that pit sites are visible—as typified by localized roug-ing, even at 20X magnification aidedby surface-scratching techniques.Therefore, in many instances in cyclicservice or batch production, SS is

briefly (i.e., 1 to several h) exposed toconditions above the CPT. Although corrosion damage is accumulated, in-tense maintenance and cleaning prac-tices may extend the service life of such equipment. However, anythingthat extends the period of exposure

above the CPT—whether larger equip-ment or an unrelated deviation in pro-

cess schedule—can lead to severe cor-rosion damage in a single cycle in batch operation.

Reference1. ASTM G150, “Standard Test Method for Elec-

trochemical Critical Pitting Temperature Testing of 

Stainless Steels,” Annual Book of ASTM Standards

(West Conshohocken, PA: ASTM, 1997).

Richard A. Corbett is a Principal CorrosionScientist at Corrosion Testing Laboratories, Inc.,60 Blue Hen Drive, Newark, DE 19713-3406. He

has been working in the fields of corrosion andmaterials engineering, materials problem solving,and failure analysis of products and structures formore than 25 years. He received a NACEDistinguished Service Award and is a Fellow of theInstitute of Corrosion, a NACE Corrosion andCathodic Protection Specialist, and a 28-yearNACE member.