effects of corrosion in woodboard industries

7/28/2019 Effects of Corrosion in Woodboard Industries

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Cherchez la Corrosion Big Effects of Little Corrosion

Alex NadezhdinNorbord Inc.

4960 Levy StreetSt-Laurent, QC, H4R 2P1

Canada

Scott McQuatNorbord Inc.

4960 Levy StreetSt-Laurent, QC, H4R 2P1

Canada

Abstract

The engineered wood industry involves production processes which are essentially dry innature. Although moisture and condensation are present in steps such as flaking, drying,blending, and pressing of the wood furnish, the effect of bulk corrosion is mostly trivial and/ornon-critical to the equipment. Instead, the secondary effects of corrosion such as wear andcavitation enhancement, or the hydrogen embrittlement of the high strength fasteners mayhave expensive consequences in terms of reliability of the overall process. The article presentsseveral examples of big impact caused by little, sometimes invisible, corrosion in thebackground.

Key words: corrosion, hydrogen embrittlement, OSB

Introduction

Structural panel, also known as OSB (oriented strand board) has been arguably the fastestgrowing segment of wood-based construction materials in North America and Europe over thelast 25 years. The main characteristics of OSB are predictable structural properties, controlledswell by moisture and the ability to use juvenile and low quality plantation wood as the feedstock. The production starts with debarked logs which are then cut into roughly rectangularflakes of 0.5 mm to 1 mm thickness, 75 to 90 mm long and various widths. The flakescontaining up to 50% w/w water are dried down to between 2 and 8% moisture, then blended

with small amounts of resin and wax. Next, the flakes are formed into a three-layered mat on acontinuous belt and pressed either continuously between two stainless steel belts or in a multi-opening stack press containing 10 to 12 or more floating platens, to which the heat is suppliedvia internal channels using thermal oil or, rarely, steam at roughly 200 C.

The overall production sequence is essentially dry, although some water comes in contact withthe equipment via cooling spray on the back of flaker discs, condensation in the dryer exhauststreams, fine spray of adhesive and wax emulsions, and the steam released from the flake mat


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upon pressing between hot platens. There is also occasionally water entering the system as amist released by the automatic fire suppression system, during fire-quenching, andmaintenance shutdown cleaning. In most cases the effects are trivial, visible and easy tohandle, with a few exceptions which are described in the paper.

Figure 1: Multi-opening OSB press assembly

The cases of important corrosion described in this document include accelerated wear in post-dryer cyclones, cavitation in thermal oil systems due to pitting caused by trace amounts ofwater and Sulfur in thermal oil, Hydrogen embrittlement of high strength fasteners in contactwith water during storage and in a nominally dry application, and the hidden oxygen-cellcorrosion set up inside continuous press rolling rods normally operating in the 150 C range.

Case Studies

Case Study 1: Accelerated Wear of Dryer Outlet Cyclone

Rotary dryers, the workhorse of the modern OSB process, are slightly inclined rotary vesselsthrough which green flakes and hot gas generated by a burner move co-currently towards theoutlet. The flow then passes to a separation step from which the dry wood is transported to thenext processing stage, while the exhaust exiting at 120 C is passed through a wetelectrostatic precipitator (WESP) and regenerative thermal oxidizer (RTO) to clean and destroyentrained organic pollutants. The wood/gas separation is often conducted in a cyclone, whichis subject to wall thinning often interpreted as wear. This notion is usually reinforced by theinevitable presence of grit and silica coming from wood that gets entrained in biomass burnerexhaust (biomass presently being a conventional source of heat for the dryers). Based on thistheory, hardened steel plate is sometimes specified for such application. The consequences ofsuch choice are unjustifiably high cost of construction and premature failure as is seen inpicture [Figure 2].


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Figure 2: The razor thin wall of a rotary dryer cyclone

The cyclones seen in figure 2, were installed at the outlet of single-pass rotary dryers in 2006

as part of a newly constructed production line. The cyclones were insulated and served withoutissues for the last 5 years, until complete penetration through the wall was apparent byescaping steam and flakes. Further inspection indicated that several square meters of the wallbecame paper thin and open to outside.

The photos in Figure 3 show both sides of a section cut from the damaged cyclone wall; theshiny metallic side being the inside surface, while the dark, oxidized side with insulation still onit is the outside. The upper edge was razor thin and cracked in several locations. The material,a structural steel with elevated content of manganese, had Vickers hardness (VHN) greaterthan 200, which is well above the ~140 VHN of the ordinary mild steel.

Figure 3: The 2 sides of thinned wall sample: on the left working surface

The clean appearance of the inside surface may suggest a mechanical erosion by hard ashcomponents (e.g. silica) making their way through the dryers. A closer examination under low(x10) magnification shows a grooved pattern compatible with abrasive wear on the insidesurface, but also, pitting of the surface as seen in Figure 4 below.

Inside cyclone

Outside surface


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Figure 4: Pitting is seen on the working surface of worn out cyclone wall

Upon closer examination, one can see multilayered pits with corrosion products at the bottom,leading to corrosion being a contributor to wall thinning in spite of shiny, rust-free

appearance. The so called erosion-corrosion mechanism basics are well described inreference.1

Replacing the wall with relatively low cost, 12% Cr stainless steel (UNS S40977) solved theproblem at a fraction of the cost required to seal the entire structure against sucking in cold air,which leads to the localized condensation and corrosion.

Case Study 2: Corrosion Accelerates Cavitation Damage in Hot Oil Channels

Modern OSB presses use thermal oil heating to activate the adhesive on the wood flakes. Theoil flows from the heater to metal press platens where the wood mat is pressed and cooked. In

this case, oil began to leak at various locations in a thermal oil pipe manifold in a newlycommissioned continuous OSB press. One of leaks occurred at an end cap of the header.The picture in Figure 5 shows a sectioned view of a failed end cap with a clearly visible holethrough the thickness of the metal.

Figure 5: Profile of the penetrated thermal oil header cap

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The image shows that there is no significant thinning on the cap wall, except a very narrowregion around the penetration itself. Initially, no thinning was detected by an ultrasonicthickness survey of the pipes in locations such as elbows, inlets, or around fittings, whichwould be likely to wear by the flow of abrasive media. However, for the specific position of theend cap relative to sharply turning oil flow provides conditions ideal for forming of highlylocalized cavitation damage sometimes referred to as a hydraulic whistle, which isschematically shown by the drawing in Figure 6:

Figure 6: Hydraulic whistle created by dead end in the thermal oil header

Examination of the damaged end cap area under the scanning electron microscope (SEM),showed:

a) Metal loss pattern indicative of mechanical action such as wear by cavitationb) Small-scale pitting inside the worn out recesses with confirmed presence of crystalline

particles containing Fe and S as main components (the signature of corrosion) in thebottom of the pits.

Figure 7: The surface of the hole at low magnification (x40) under SEM; pitting seen in themiddle of a coarser wear pattern

Regionofreducedpressure


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Figure 8: Corroded sites inside the hole at high magnification (x1500); elemental analysisshowed presence of Sulfur

The metallographic cross-section of the end cap hole seen in Figure 9 shows a normal ferritic-pearlitic microstructure with generally smooth and slightly compressed edge, indicative of theerosion factor. The presence of a closed type pit in the main hole surface confirmed localcorrosion attack caused by thermal oil impurities, most likely Sulfuric acid. In other words,corrosion-aided erosion was identified as a mechanism of wall penetration.

Figure 9: Metallographic cross-section of the hole: smooth, compressed edge with a closed-type pit in it

Replacing the failed caps with UNS S30400 eliminated leaks in the short term, but replacingthe Tee type ends with a round elbow allowed the system to run problem free for the last 10years. In other words, corrosion became non-essential when special hydrodynamic conditionsceased to exist.


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Case Study 3: Hydrogen Embrittlement of Fasteners in Wet Service

This next case deals with the use of high strength fasteners in dynamically loaded productionequipment. The so called hydrogen embrittlement (HE) of carbon and low alloy steel fastenersis part of the more general case of stress-corrosion cracking, which is driven simultaneously bystress and corrosion factors.2 Some authors only allow using HE terminology for the cases

related to fabrication process, which could include pickling and electroplating, the rest beingclassified as embrittlement in service, being a part of the more general stress corrosioncracking phenomena.3 In this text HE is used for the embrittlement in service for brevity. Atypical feature of such mechanism is the presence of branching, i.e., secondary cracking whichcrosses the main plane of fracture surface. The HE mechanism is normally interpreted as theresult of atomic hydrogen, a primary product of steel surface corrosion by acid (including sucha weak one as water), migrating into the bulk of metal and forming pockets of elementalhydrogen gas at impurities or grain boundaries inside the metal micro-structure. Those pocketspush metal grains apart forming non-connected microcracks capable of coalescing into largercracks under stress applied in service. An early sign of such embrittlement, beside microscopicobservation is the reduction in maximum elongation value as compared to the one specified for

a given material.

In the OSB industry, thin wood flakes are made from debarked logs by a machine based onfast rotating forged steel head with sharpenable or disposable knives placed in heavy pie-likecarriers (in one of the two predominant designs). A carrier is attached to the disc via a set ofhex-cap screws made from Dacromet(1) coated SAE(2) Grade 8 high strength bolts. The face ofthe disk sprayed with water as coolant during cutting creates a possibility of exposing thescrews to moisture on a semi-permanent basis. Many precautions are used to prevent HE fromhappening in service, such as the protective coating, a tight fit, a water-barrier polymer grease,and a regularly scheduled fastener replacement.

Despite of prescribed precautions, a catastrophic failure occurred when one of the carriersbecame loose and separated from the disc triggering the separation and flying off of all othercarriers around the circumference of the disc. The investigation began by collecting all the capscrews (8 per carrier) broken in the process with some of them still lodged in the disc. In fullaccordance with the described failure, the vast majority of the broken screws displayed pureshear fractures. Nevertheless, some of the screw fragments lodged in the disc and scatteredaround, had a distinctly grainy fracture surface with well pronounced beach marks, indicatingan alternative failure mechanism, which might have taken place before the overall flakerfailure. While several carriers had one or 2 fasteners of the latter kind, one of the carriers had 6screws broken this way. We were quite confident that this particular carrier was the first one toseparate.

Pictures taken from the third party investigation report show the appearance of the describedfracture surface after cleaning.4 It shows a characteristic pattern which is incompatible witheither shear or fatigue failure in spite of the presence of ratchet and beach marks.

(1)Trade name

(2)Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, PA 15096-0001.


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Figure 10: Fracture surface after cleaning. Black rectangle marks the area shown at highermagnification under SEM.

Upon closer examination under SEM, characteristic features of HE were revealed andcorrespond with the discovery of prolonged contact of bolts with water due to the absence ofwater-proof tight fit between the carrier and the disc surface and skipping of a scheduled boltreplacement time.

Figure 11: Closer view of the rectangular area in Figure 10; intergranular separation isconsistent with HE mechanism


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Case Study 4: HE Prior to Service

Unfortunately, HE of high strength fasteners may occur even before they are placed indemanding service. At one point, a set of hex-cap screws, similar to those described in thepreceding case started snapping one by one only a few days after installation in a flaker disc.The fracture surface showed previously observed HE-type appearance, in spite of the absence

of visible corrosion and virtually intact corrosion-protective coating. The metallographicexamination of a polished axial cross-section of the fastener revealed the true condition andsome of the history of this batch of fasteners prior to shipment. The photos below clearly showa micro-crack crossing an inclusion inside the metal structure (sign of embrittlement), and a pitin the surface of the fastener. The pit is partially filled with the protective coating materialindicating that the pit had formed prior to coating. In other words, surface corrosion (the causeof HE) has taken place during the period between original fabrication and protective coatingbefore shipment. This could have been the result of storage under conditions allowing moisturecondensation or a pickling/cleaning step not followed by an adequate bake out to purge theHydrogen.

Figure 12: HE of high strength bolt prior to service

Case Study 5: HE of High Strength Fasteners in Dry Service

Pressing of wood in panel board industries is mechanically the most strenuous operation whichrequires high pressures across large flat areas at elevated temperatures. This is oftenachieved by using the multi-opening stack presses where suspended platens bound bysimultaneous arms to move up and down in synchronous manner are being pressed together

with wood mat between each pair. The stress is ultimately applied against a stationary platenwhich transfers the load to the frames. Over the period of several decades, the press designhas undergone substantial evolutions going from columnar to welded frames construction andending up with the modern arrangement of bolted construction which relies on the strength andstiffness of high grade bolts used all around the frame and simultaneous arms.

Normally, such bolts are not subject to side bending and by far exceed the strength limitationsimposed by working stresses. So, when the first failures were discovered after a few years in


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service, a fractography and metallographic study was completed in order to determine thefailure mechanism. First of all, in spite of generally shiny metallic appearance of the first failedbolts, tiny pits seen under the microscope suggested a surface corrosion attack, most likely bymoisture condensed from generally humid atmosphere around the press.

The appearance of the fracture surface indicated a bending component and ratchet marksaround the perimeter. The in depth microscopy study, including SEM to search for signs of

fatigue, showed that although there were some small area showing striations, the majority ofthe fracture surface showed evidence of branched cracking crossing the plane of main crackpropagation. In some cases this ancillary cracking could be seen even under optical stereomicroscope and on the polished metallographic cross-section. The discovery of branchedcracks perpendicular to the main crack are clear signs of stress-corrosion, and in this case,hydrogen embrittlement mechanism.

.

Figure 13: The metallographic cross section perpendicular to the fracture surface is showingsecondary cracks suggesting partially intergranular propagation mode.

Once again, the HE is seen as a contributor to failure mechanism, despite the minimalexposure to moisture due to the high temperature of metal surfaces during pressing.Nevertheless signs of surface corrosion were found on the heads and other locations, whichcould have been the result of periodic condensation during weekly down days.

Case Study 6: Hidden Corrosion behind Oil Plug

Our final case study describes the severe corrosion which took place in a closed cavity behind

an oil plug in a moving part which normally operates at temperatures well above the boilingpoint of water. In a continuous OSB press the wood mat is inserted between two continuousstainless steel belts through which pressure and heat are transferred from hot platens. Thelubricated rolling rod carpet between the platen and the belt allows for belt movement over thestationary platens.

The roller rods are made from medium carbon steel in the annealed condition and casehardened on their surface to withstand the high pressures exerted in the press. Each 18 mm


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diameter rod has a 9 mm hole drilled out on each end to a depth of about 140 mm in order toinsert lubricated pins attached to the moving side-chains which run parallel to the belt at halfspeed. Although there is minimal interaction between the pins and the roller rods, the pins areconstantly lubricated by using specialized high temperature oil. Throughout the life of thecontinuous press, the roller rods have failed periodically by fatigue initiated from inside due toseveral different mechanisms. To our surprise we have found that one of them has involvedmassive corrosion pitting of the cavity behind the pin and a densified oil plug formed due to

thermal oil deterioration and wood dust accumulation in service.

Figure 14: Roller rod carpet being evaluated for end cavity wall thickness by using ultrasonicthickness gage

The appearance of pitted surface seen in the axial cross-section of the roller rod (Figure 15)leaves little doubt about the conductive liquid participating in the attack, in this case, liquidwater, which penetrated the cavity behind the pins at one time in service history.

Figure 15: Axial cross-section of the rod ends with pitting clearly seen at the ends of the rodcavity


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Considering the rods and pins operate at temperatures of >150 C, much exceeding the boilingpoint of water, the only possibility for this to happen would be during extended shutdownfollowing pressure water wash, which indeed has been practiced at the plant. The presence ofsticky oil plug blocking access to fresh air had greatly exacerbated the corrosion process viathe so called oxygen concentration cell,5 where the relatively small area of wetted steel underlow oxygen atmosphere played the role of anode of the galvanic cell, while the cathode was

the rest of the roller rod surface fully exposed to air. Pitting at the diameter change betweenthe end of cavity and the solid rod has been a natural fatigue starter. Once again hiddencorrosion taking place during prolonged shut down of the press has led to a failure of criticalmetal parts running in an essentially dry production process.

DISCUSSION

The main objective of this presentation has been to give an outline of various cases of liquidcorrosion in a predominantly dry production process. Many associated issues could constitutea subject of separate papers, such as, for example the selection of exhaust gas ducting andfan design and materials, corrosion of ceramic bed in environmental control devices, stress-

corrosion cracking of stainless wet electrostatic precipitators, deterioration of mill chains andso on.

While being rather mature in terms of optimized production processes and equipment, thepanel board industries still have some room to progress towards rational design andproduction conditions aimed at minimizing costly corrosion damage. While high operatingstresses and cyclic nature of many operations of the panel board production tend to focusoperators and designers attention on mechanical issues, first of all wear and fatigue, takinginto account potential contribution of wet corrosion during operation and shut down periods isexpected to substantially reduce frequency and severity of failure.

CONCLUSIONS

Mechanical wear in wood drying operations is often the result of corrosion-erosion due tomoisture condensation in post-dryer ducts and cyclones.

Sometimes the corrosion contribution with important consequences can be detected only byusing powerful magnifying devices such as SEM analysis.

Selecting of high-strength fasteners, however advantageous it would be for achieving thedesign objectives needs to be critically analyzed in terms of possibility of the in-serviceembrittlement even under nominally dry service conditions.

Pressurized water cleaning of the production equipment which normally operates in an oilyenvironment needs to be reviewed from the perspective of potential to create pockets of waterin contact with metal surface, which under conditions of limited fresh air access would formaggressive oxygen cells destroying equipment literally from inside out.


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REFERENCES

(1) Rihan, O.R., Nesic, S., Erosioncorrosion of mild steel in hot caustic, Corrosion Science48 (2006) 26332659

(2) Hendrix D. E., Hydrogen embrittlement of high-strength fasteners in atmospheric service,Materials performance Y. 1997, vol. 36, No. 12, pages 54-56

(3) Sisson, R., Hydrogen Embrittlement and Electroplating, Fastener TechnologyInternational, August 2001, 50-51

(4) Acuren confidential report A07605-03479-01-02R0-60CAS006

(5) Concentration Cell Corrosion, Corrosion Technology Laboratory, n.d. [accessed 2012 July20]; Available from: http://corrosion.ksc.nasa.gov/conccor.htm

effects of corrosion in woodboard industries

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