Weathering durability ofchromium-treated southern pine

William C. FeistR. Sam Williams

AbstractThe purpose of this study was to compare the efficacy

of chromated copper arsenate (CCA) and chromium tri-oxide treatments in decreasing the weathering of unfin-ished wood and improving the durability of semitranspar-ent and solid-color stains applied to the treated wood.Unfinished and finished southern pine sapwood specimenstreated with either CCA or chromium trioxide were ex-posed to accelerated weathering. Small amounts of chromi-um salts on the wood surface greatly decreased weathering(erosion) of the wood caused by ultraviolet-light-catalyzeddegradation. For unfinished wood, pressure treatmentwith CCA provided long-term protection against erosion.Treatment of the wood surface with a brush-applied solu-tion of chromium trioxide also decreased erosion but notto the extent found for the wood treated with CCA. Forfinished wood, the CCA treatment greatly extended thelifetime and durability of a partially ultraviolet-light-transparent (semitransparent) stain. Stain durability onwood pretreated with CCA under pressure was superior tothat on wood pretreated with brush-applied chromium tri-oxide; the CCA treatment penetrated the wood and de-posited more chromium at the surface than did the chrom-ium trioxide surface treatment.

Much of the wood treated with chromated copper ar-senate (CCA) preservative is used outdoors for numerousarchitectural purposes (14). The CCA-treated wood is espe-cially popular for decks, patios, and retaining walls. Rossand coworkers (10,14) found that treated wood is often fin-ished with clear or pigmented coatings for esthetic effectsand for protection of surfaces from water. Protection fromwater is necessary to minimize shrinking and swelling,thus minimizing checking, cracking, and other deforma-tions.

Clear and lightly pigmented finishes (such as semi-transparent stains) permit exposure of the wood surfaceto some ultraviolet (UV) light. As this light degrades thesurface of the wood, the durability of the finish is decreased.The durability of wood stained with semitransparent stain

parallels that of unstained wood. However, wood surfacescan be stabilized against UV-light-catalyzed degradationusing a variety of chemical treatments (1,3,5,6,11,18-21).Such stabilization greatly increases the durability of semi-transparent stains. Some chromate-containing pretreat-ments have been shown to more than double the servicelife of semitransparent stains on both hardwoods and soft-woods (5,6). In preliminary work at the Forest ProductsLaboratory (15), white fir specimens that were pressuretreated with chromated-copper-boron preservatives weath-ered more slowly than did untreated controls. The dura-bility of a semitransparent stain applied over the woodtreated with chromated-copper-boron preservative was im-proved as compared to that of the stain applied over un-treated wood.

Previous work showed that application of aqueous so-lutions of hexavalent chromium (e.g., chromium trioxide)to wood surfaces inhibited and retarded wood erosion dur-ing outdoor weathering and enhanced the life of surfacefinishes applied over the treated wood (5,8,21). Reactionsof hexavalent chromium compounds with wood resulted ina wood surface enriched in chromium(III) bonded to com-ponents of the wood cell wall. Reactions involved oxida-tion of the surface wood components by chromium andsubsequent fixation of the reduced chromium(III) to lig-nin and possibly to wood carbohydrates. The chemical mod-ification of the wood and the presence of chromium(III)at the wood surface played a role in the observed resist-ance to weathering and in the protection from UV light(19-21). Because the commonly used CCA Type C preserv-

The authors are, respectively, Supervisory Research Chem-ist and Research Chemist, USDA Forest Serv., Forest Prod. Lab.,One Gifford Pinchot Dr., Madison, WI 53705-2398. The authorsgratefully acknowledge the laboratory assistance of Peter G. So-tos, Mark Knaebe, and David I. Gutzmer, and the scanning elec-tron microscope assistance and advice of Thomas A. Kuster. Theuse of trade or firm names in this publication is for reader in-formation and does not imply endorsement by the U.S. Depart-ment of Agriculture of any product or service. This paper wasreceived for publication in March 1990.

Forest Products Research Society 1991.Forest Prod. J. 41(1):8-14.

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ative consists of approximately 47 percent (by weight)chromium trioxide, a similar effect for UV light protec-tion should be expected for the chromium/wood componentinteractions in CCA-treated wood. This indicates that sur-face finishes applied over CCA-treated wood should haveenhanced performance as compared to finishes appliedover untreated wood (8).

The purpose of this study was to compare the efficacyof chromium trioxide and CCA treatments in decreasingthe weathering of unfinished wood and improving the du-rability of semitransparent and solid-color stains appliedto the treated wood.

Materials and methodsWood specimens

Planed sapwood specimens of southern pine (Pinus sp.),75 by 9.3 by 100 mm (longitudinal), were used to deter-

Figure 1. — Drawing of nominal southern pine board showing45° angle of growth rings to surface and selection of 75- by100- by 9.3-mm specimens from top planed surface of the board.

mine weathering rates and finish performance. The spec-imen surfaces (75 by 100 mm) exposed to accelerated weath-ering were 45° to the radial and tangential wood surfaces(Fig. 1). For convenience, these oblique surfaces are refer-red to as flat grained. These small test specimens werecut from nominal 50 by 100 by 457 mm (longitudinal)southern pine lumber. Each board yielded a replicate foreach of the finished and unfinished groups. Three repli-cates cut from different boards were used for each eval-uation. The replicates were cut from untreated wood orfrom wood-pressure treated with CCA chemicals. The outerplaned surface of the original large board was used as theouter surface of the test specimen (Fig. 1).Pretreatment and finishes

Two pretreatments were used: (a) pressure treatmentwith CCA chemicals using the full-cell process (17) to aretention of 4.00 kg ± 0.8 kg/m3 (0.25 ± 0.05 lb./ft.3) and(b) brush application of 5 percent (by weight) aqueouschromium trioxide solution. Stains used for the untreated,CCA-treated, and chromium-trioxide-treated southern pinespecimens were (a) none (control), (b) oil-based, semitran-sparent stain (1), and (c) solid-color acrylic latex stain.Characteristics of the stains and pretreatments are shownin Table 1.

Chromium-trioxide-treated specimens were heated inan oven at 135°C for 10 minutes to fix the chromium saltsin the wood (5). The stains were applied by brush undernormal laboratory conditions to specimen surfaces thathad been wiped with a soft cloth, washed with distilledwater, and conditioned to 65 percent relative humidity(RH) at 27°C. Stain was applied to all surfaces except theback while the specimens were in a horizontal position.The amount of chromium trioxide or stain applied to thesurface was determined by weighing, and spreading rateswere calculated (Table 2).

After stain was applied to the specimens, the specimenswere conditioned at 60 percent RH and 21°C for 1 weekto assure complete curing. Specimens were then placedinto stainless steel holders that protected the upper one-third of the surface during exposure in an acceleratedweathering chamber. Erosion of the flat-grained wood sur-faces was measured optically after exposure to light at600-hour intervals, using a calibrated optical microscopeat a magnification of 100× (9,16).Accelerated weathering

Accelerated weathering was conducted in a commercialchamber consisting of high-intensity 6500-W xenon arclight exposure for 24 hours per day; the procedure included

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Figure 2. — Earlywood erosion of untreated, chromium-trioxide(CrO3)-treated, and CCA-treated southern pine, unfinished orfinished with a semitransparent stain (ST), after 2,400 hoursof accelerated weathering. Wood erosion expressed in micro-meters per hour of exposure to light. Data for untreated, ST,and CrO3 are mean erosion values for each of three replicatespecimens. The line for ST stain was plotted on the basis ofthe least-squares fit of data excluding the 2,400-hour data. Thevariability in data for CrO3 + ST stain, CCA, and CCA + STstain is not shown, but it was similar to that shown for the un-treated and ST stain.

a 4-hour distilled water spray each day. Chamber condi-tions were 45 ± 2°C and 50 ± 2 percent RH. Acceleratedweathering was chosen instead of outdoor weathering be-cause it is reproducible and controllable and has good cor-relation with natural weathering of unfinished wood orwood finished with semitransparent stains (2,9). As mea-sured by erosion of an unfinished wood surface, 2,400 hoursof accelerated weathering corresponds to 4 to 5 years ofnatural weathering when the specimens are exposed ver-tically on a wall facing south in Madison, Wis. (9).

Figure 3. — Southern pine finished with semitransparent stainafter 2,400 hours of accelerated weathering; left, untreatedwood; center, wood brush-treated with 5 percent chromium tri-oxide (CrO3) solution before finishing; right, CCA-treated wood.

Evaluation of finishesThe American Society for Testing and Materials (ASTM)

standard developed to evaluate paint erosion was used toevaluate stain erosion. This evaluation (ASTM D 662-86)is based on pictorial standards of the Federation of Soci-eties for Coatings Technology (4). The rating uses a scaleof 10 to 1; 10 indicates no change from the original un-weathered condition, 5 indicates 50 percent erosion, and1 indicates 100 percent erosion.

Scanning electron microscopySelected specimens, both unweathered and exposed to

accelerated weathering, were mounted on 9-mm-diameterholders with a mixture of silver paint and cellulose ac-etate cement. The stub holders were then transferred toa high-vacuum evaporating unit and coated with 10 to 20nm of gold. The specimens were examined with a scan-ning electron microscope (Jeol Scanning Microscope, JSM-840, Jeol USA, Inc., Peabody, Mass.) at 15 kV and a Tra-cor Northern (TN 5500, Tracor Northern, Middleton, Wis.)energy-dispersive spectrometer.

Results and discussionWood erosion

Statistical analysis of wood erosion (weathering) dataindicated a linear relationship, and a least-squares fit gavethe erosion rate or slope in micrometers of erosion per hour(Table 3). Plots for the earlywood erosion of unfinished orfinished southern pine pretreated with chromium trioxideor CCA are shown in Figure 2. The erosion rates for late-wood were much lower than those for earlywood but fol-lowed similar trends. The greatest erosion was observedfor the untreated wood. The surface-applied chromiumtrioxide treatment decreased wood erosion by approxi-mately 40 percent (from 0.116 to 0.070 µm/hr.); the CCAtreatment decreased wood erosion by 77 percent (from0.116 to 0.027 µm/hr.). This decrease in erosion after weath-ering is directly related to the chromium concentrationat or near the wood surface (5).

Erosion of chromium-trioxide-treated wood under thestain was decreased during the early stages of acceleratedweathering because semitransparent stains are partiallytransparent to UV light (7). However, the final overall ratesof erosion for wood with no pretreatment or stain (0.116

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µm/hr.) and wood with just semitransparent stain (0.078µm/hr.) were nearly the same (Table 3). When the stainwas applied over CCA-treated wood, wood erosion was de-creased by 64 percent, compared to the erosion of staineduntreated wood (0.078 µm/hr. for ST stain and 0.028 µm/hr.for CCA and ST stain). Thus, surface stabilization againstUV light degradation from the chromium in the CCA for-mulation greatly improved wood durability for untreatedwood and wood finished with a semitransparent stain.

The improvement in durability of stain on CCA-treatedwood compared to that on untreated wood is calculated onthe basis of the least-squares fit of the erosion data at 600,1,200, and 1,800 hours of weathering. The 2,400-hour datawere excluded because microscopic examination of thespecimens at various intervals during weathering indi-cated a more rapid loss of pigment at about 1,800 hours.The loss of pigment exposed more of the wood surface toweathering and might explain the more rapid erosion from1,800 to 2,400 hours. This nonlinear behavior of thestained wood is the subject of continuing investigations.The finished wood that was pretreated with chromiumtrioxide or CCA had not eroded enough after 2,400 hoursto show this nonlinear behavior.Stain erosion

The effects of chromium surface stabilization on UVlight degradation and subsequent erosion of the semitrans-parent stain after accelerated weathering were similar tothe effects of wood erosion (Fig. 3). Stain erosion was eval-uated using ASTM visual standards (Table 4). The stainerosion values determined by this method substantiatethe microscopically determined wood erosion values. Thesemitransparent stain erosion rating after 2,400 hours was7.0 for CCA-treated specimens, 4.7 for chromium-trioxide-treated wood, and 2.3 for untreated wood. These ratingsshow that stabilization of the wood surface decreased stainerosion significantly. The decreased stain erosion of CCA-treated wood compared to that of chromium-trioxide-treatedwood reflects the greater quantity of chromium in the woodwith CCA treatment. The chromium-trioxide treatmentwas on the surface of the wood and penetrated only a fewwood cells. As the UV light degraded the wood, the woodsurface was washed away along with the chromium. WithCCA treatment, the chromium was dispersed throughoutthe wood, providing a much higher degree of protection.As was observed with wood erosion after accelerated weath-ering, surface stabilization against UV light degradationresulting from chromium in the CCA formulation greatlyimproved the durability of the stain and reduced stainerosion, thus extending the lifetime of the stain.

The solid-color stain (a pigmented, film-forming finish)applied over untreated and CCA-treated wood showed littleerosion because this stain is opaque to UV light, and thusthe wood beneath the stain does not degrade (Table 4).The degradation mode primarily involved erosion of the

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Figure 5. — Scanning electron micro-graphs of flat-grained surfaces of un-treated southern pine (a) before exposureto accelerated weathering; (b) after 2,400hours of accelerated weathering; (c) withsemitransparent stain — before exposureto weathering, and (d) with semitranspar-ent stain — after 2,400 hours of weath-ering. Bar = 10 µm.

Figure 6. — Scanning electron micro-graphs of flat-grained surfaces ofchromium-trioxide-treated southern pine(a) before exposure to accelerated weath-ering; (b) after 2,400 hours of acceleratedweathering; (c) with semitransparent stain— before exposure to weathering; and(d) with semitransparent stain — after2,400 hours of weathering. Bar = 10 µm.

surface of the solid-color stain, not the wood; any stabiliz-ing effect of the chromium on the wood surface is unim-portant with regard to the performance of the stain. Theperformance of solid-color stain applied to chromium-trioxide-treated wood was not evaluated.

Chromium analysis by energydispersive spectroscopy

The difference in penetration of the chromium salts inthe two pretreatments can be seen by comparing energydispersive spectroscopy spectra of cross sections of chromi-um-trioxide- and CCA-treated specimens after acceleratedweathering (Fig. 4a). The chromium in the specimen treatedwith brush-applied chromium trioxide was at or just be-low the wood surface, whereas the chromium in the CCA-treated wood extended through the depth measured in thespecimen. The untreated control shows only a weak sig-nal along the baseline, with a small amount of background

noise. Figure 4b is a scanning electron micrograph of across section of a weathered specimen, with scan line in-dicated.

Microscopic wood degradationScanning electron micrographs (Fig. 5) of the unfin-

ished, untreated southern pine specimens before acceler-ated weathering (a) and after 2,400 hours of acceleratedweathering (b) showed the expected degradation of pitsand middle lamella, and formation of checks and voids(3,8,12,13). Encrustations are visible on the unexposed,untreated wood surface of specimens finished with thesemitransparent stain (Fig. 5c); the corresponding degra-dation of wood cells beneath the stain after weatheringis also shown (Fig. 5d). Some stain particles remained onthe exposed surface. The partial UV light transparencyof the semitransparent stain permitted some degradationof the wood substance underneath the stain.

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Wood surfaces brush-treated with 5 percent chromiumtrioxide solution appear similar to untreated surfaces be-fore weathering (Fig. 6a) because the treatment completelypenetrated the wood surface. The amount of wood degrada-tion after accelerated weathering was less than that ob-served for the untreated wood (Fig. 6b), demonstrating thepartial UV-light-stabilizing effect of the chromium treat-ment. However, chromium was slowly lost, and increasedwood degradation, particularly of pit structures, was ob-served. Figure 6c shows the chromium-trioxide-treatedwood before weathering and Figure 6d shows the greaterretention of semitransparent stain encrustations on thechromium-trioxide-treated wood surfaces after weather-ing, which reflects the improved resistance of the under-lying wood to UV light degradation.

Some wood surface degradation was observed for CCA-treated wood (Figs. 7a and 7b). However, those surfaceswere generally less eroded and showed less checking thanthose of untreated wood; some pit deterioration was ev-ident. Figure 7c shows CCA-treated wood before weath-ering and Figure 7d shows the markedly improved dura-bility of the semitransparent stain on the CCA-treatedwood after accelerated weathering. More stain particleswere retained on the wood surface, and wood degradationbeneath the surface was greatly reduced compared to un-treated and chromium-trioxide-treated wood.

Concluding remarksThe presence of chromium in sufficient concentration

on the wood surface greatly decreased the rate of wooderosion caused by UV light degradation. For unfinishedsouthern pine sapwood that was pressure treated withCCA, the nearly complete penetration of the chemicalsprovided long-term protection against erosion (weather-ing) because the chromium was distributed throughout thewood. Brush treatment of unfinished wood specimens withaqueous solutions of chromium trioxide also decreasedwood erosion but not to the extent found for CCA-treatedwood, Because the chromium trioxide treatment did not

Figure 7. — Scanning electron micro-graphs of flat-grained surfaces of CCA-treated southern pine (a) before exposureto accelerated weathering; (b) after 2,400hours of accelerated weathering; (c) fin-ished with semitransparent stain —before exposure to weathering; and (d)finished with semitransparent stain —after 2,400 hours of weathering. Bar =10 µm.

penetrate the wood cells deeply, chromium was not presentthroughout the wood; after the surface chromium erodedaway, the wood deteriorated.

The CCA treatment also greatly extended the life anddurability of a partially UV-light-transparent (semitrans-parent) stain applied to the treated wood surface becausethe chromium stabilized the wood surface against UVlight degradation. The durability of semitransparent stainon CCA-treated wood was much better than that of stainon wood that was brush-treated with chromium trioxidesolution, which demonstrates the enhanced stability ofCCA-treated wood. As long as chromium was present inthe wood cell wall, the wood surface was stabilized andthe durability of the semitransparent stain was thus im-proved.

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