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Corrosion: Corrosion


April 1, 2002
By Pulp & Paper Canada

EXTERNAL STRESS CORROSION CRACKING OF STAINLESS STEEL VESSELSWet thermal insulation may be hiding serious cracking problemsLurking beneath wet thermal insulation could be a serious threat to the safe …

EXTERNAL STRESS CORROSION CRACKING OF STAINLESS STEEL VESSELS

Wet thermal insulation may be hiding serious cracking problems

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Lurking beneath wet thermal insulation could be a serious threat to the safe operation of solid stainless steel vessels and tanks, external stress corrosion cracking (ESCC). Figure 1 shows a case of severe ESCC of an oxygen reactor that was constructed using solid type 316L stainless steel. Over the past decade or so many pressure vessels were constructed using solid austenitic stainless steels (such as types 304L and 316L) in attempts to eliminate process-side corrosion problems. These vessels include oxygen reactors, concentrators, evaporators, steaming vessels, and tanks. Little thought was given to the possibility that cracking problems could develop on the outside surfaces.

For stress corrosion cracking (SCC) to occur three prerequisites must be met: a susceptible material, an environment containing an SCC agent such as chloride ions, and tensile stress. It has long been known that the conventional 300-series austenitic stainless steels are particularly susceptible to chloride SCC. Chlorides are ubiquitous in pulp and paper mill environments and are even present in small amounts in fresh water and rainwater. Some thermal insulation can contain chlorides. When water or process fluids gain access to hot surfaces they evaporate, concentrating even small amounts of chlorides to levels that can support SCC on susceptible materials provided that tensile stresses are present. Forming stresses for stainless steel vessel sections such as heads and cylindrical rings result in external surfaces being put into tension. Welding adds additional residual tensile stresses.

Water or process fluids can gain access through almost any insulation jacket, particularly if the jacket has been damaged and left unrepaired. The insulation holds the liquid against the hot external surface of the vessel as it slowly evaporates and concentrates the chloride ions. The high tensile stresses on the outside of the vessel support the initiation and growth of ESCC. A characteristic of SCC and ESCC is that the cracks branch as they grow. A single crack on the surface may resemble the roots of a tree below the surface.

The nondestructive testing technique most available at pulp and paper mills is penetrant testing (PT). Acoustic emission testing is also useful but requires specialized equipment. Light grinding is necessary for the detection of cracks on external surfaces by PT. Grinding exploration will inevitably reveal more extensive cracking, giving rise to the fallacious belief that grinding “causes” the cracking. Eventually, however, the cracking bottoms out. Depending on the numbers and depth of the excavations the vessel may or may not be ft for continued service (the oxygen reactor in the Figure was condemned). There is no satisfactory weld repair for ESCC.

If it is possible to remove the cracks by grinding that does not violate the pressure boundary, coating of the vessel with an immersion grade coating is recommended before the insulation is replaced. NACE International has published guidelines for such coatings. There are also spray-on ceramic coatings available that combine both thermal insulation and corrosion protection.

Where it is not possible to remove the cracks, de-rating or the partial or complete replacement of the vessel may be required. For partial replacements it is suggested that any cut line be made at least 30 cm from the last detected location of ESCC. New vessels can be constructed using duplex stainless steels such as UNS S32205 that are more resistant to SCC and ESCC.P&PC

CRACKING OF PRIMARY AIR PORT OPENING COMPOSITE TUBES IN RECOVERY BOILERS

Cracks in composite tubes at primary air ports do not necessarily stop at the stainless-carbon steel boundary

In recent years, serious cracking problems have been found in the primary air port opening tubes in numerous recovery boilers worldwide. The cracking can have either a “mosaic” (craze-like or “spiderweb”) appearance or can be linear (usually circumferentially-oriented). Figures 2 and 3 show examples of both forms of cracking.

The mosaic cracking normally is confined to the stainless steel cladding of the composite tubes and does not seem to pose as serious a threat to the integrity of the tubes as does linear cracking. Examination of tubes having linear cracking that were removed for metallurgical analysis has confirmed that linear cracking can continue to propagate into the carbon steel. There has been one ESP resulting from a leak in a tube adjacent to a primary air port opening.

Primary air port opening tube cracking appears to be more widespread in boilers having castings in the air port openings. The cracks in these boilers are normally occur at the lower half of the opening, affecting the tubes on either side from a distance of 150 mm below the termination of the membrane to 500 mm above the membrane termination, extending upwards in a broad arc across the face of the tubes and sometimes into the adjacent straight tubes. In recovery boilers having welded nozzles in the primary air port openings the cracking is normally in the nozzle welds. Weld cracking has propagated into the adjacent composite tubes.

It is important that proper nondestructive testing be done to detect cracking at primary air port opening tubes. It cannot be over-emphasized that conventional penetrant testing (PT) where the tubes surfaces are prepared using wire brushes is inadequate for detection of the cracking. The cracks in most primary air port opening tubes are either too tight or filled with deposits that prevent their detection by conventional PT. The preferred surface preparation for PT is cleaning the tubes using 120-grit flapper wheels or sanding discs. Only competent nondestructive testing companies should be used for such a critical inspection.

Removal of the cracks by grinding followed by weld repairs using nickel-base filler metal has been an effective repair procedure. However, the residual stresses associated with welding often causes previously undetected cracks to open up and accept penetrant, or promotes the initiation of new cracks in service. If tube replacement is necessary, nickel-base alloy composite tubes are known to have a superior resistance to cracking in a number of recovery boiler lower waterwall environments including floor tubes and smelt spout opening tubes.P&PC

RAPID CORROSION THINNING IN CONTINUOUS DIGESTERS

The unpredicted onset of rapid thinning can result in expensive unscheduled repairs and downtime

For many years there was no problem with corrosion thinning of the carbon steel walls in most kraft continuous digesters worldwide. Beginning approximately 10 years ago, the numbers of reports of rapid corrosion thinning began to increase. Such thinning is often characterized by a bright “active” appearance of the digester wall as seen in Figure 4. In the early 1990’s, the phenomenon of rapid thinning was not well understood. Today, we have a much better understanding of the causes of rapid thinning yet cases continue to occur and many of these cases were as unexpected as they were unwanted.

In conventional digesters, rapid corrosion thinning occurs most often in the wash zone below the extraction screen (regardless of whether or not the wash screens are present). Rapid thinning can also occur behind the extraction screens and in the rings above the extraction screens. In more modern digesters, the rapid corrosion thinning is observed between the extraction and cooking screens and between the cooking and wash screens. While the TAPPI Digester Corrosion Task Group defines rapid thinning as a corrosion rate greater than 0.5 mm/year, there have been cases of rapid thinning at rates approaching 6 mm/year. Considering that most continuous digesters were constructed having a corrosion allowance of 6 mm, undetected rapid corrosion thinning can result in the pressure boundary being thinner than that required by the construction code.

It may be necessary to perform more extensive nondestructive testing
of the digester wall to ensure that the vessel is safe for continued operation or to identify areas that are below code-minimum wall thickness values. Ultrasonic thickness testing on a 0.5 m x 0.5 m grid has been useful for this purpose. Weld buildup with carbon steel is only suitable for restoring wall thickness and is not a repair for rapid corrosion thinning since carbon steel welds are even more susceptible to rapid thinning than is the plate from which the digester was constructed.

Investigations into the corrosion of carbon steel in continuous digester extraction and wash zone liquors have revealed two parameters that correlate well with rapid thinning: residual hydroxide concentration and temperature. High corrosion rates are measured in liquors where the residual hydroxide concentration is very low (less than 8 g/L expressed as NaOH) or where the temperature is high. Rapid corrosion has been measured at temperatures above approximately 165C (330F) in softwood liquors and 150C (304F) in hardwood liquors. The extractives from different wood species may vary considerably in their corrosivity so it is difficult to make broad generalizations as to the conditions under which rapid corrosion thinning will occur. The trend in kraft cooking is to lower the hydroxide residual levels and to increase temperatures in the wash zone of the digester.

At present there are three alternatives for protecting of continuous digesters from rapid corrosion thinning. These are: stainless steel weld overlay, anodic protection systems, and thermal spray coating. The selection of the best alternative must be decided on a case-by-case basis.

Dr. Angela Wensley, Pulp and Paper Corrosion Specialist, Angela Wensley Engineering Inc., White Rock, BC


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