.
Corrosion of Metal in Wood Products A. J. Baker
1
Corrosion of Metal
in Wood Products
REFERENCE:
Baker, A. J., Corrosion of Metal In Wood Products, Durability of
Building Materials and Components. ASTM STP 691. P. J. Sereda and G. G. Litvan,
Eds., American Society for Testing and Materials, 1980, pp. 981-993.
ABSTRACT:
A description is given of the source of metal corrosion products that cause
wood deterioration around corroding metal in wet wood. Corrosion of isolated steel
fasteners in wood and the formation of acid conditions around the corroding steel is ex-
plained in terms of crevice corrosion. The source of acid and alkaline conditions
around corroding metals undergoing galvanic corrosion and the source of alkaline condi-
tions around cathodically protected metal also are explained.
Results of a 3-year corrosion test of eleven wood fastener materials in wood treated with
copper-containing waterborne salt preservatives indicate that. when long service life is re-
quired under wet conditions, fastener materials that are cathodic with respect to copper
should be chosen. The literature indicates that metals in contact with fire retardant-
treated lumber and cellulose insulation under high moisture conditions also may be sub.
ject to corrosion.
KEY WORDS: metal corrosion. corrosion damage. wood preservatives, wood products.
cellulose. insulation, galvanic corrosion, fire resistant coatings, cathodic protection,
building materials, durability
It is commonly observed that wood weakens around corroding metal such
as nails. spikes. screws, bolts, and plates in house siding, rail ties, boat
docks, highway trailer beds, and wooden vessels. Damp wood not only causes
metals to corrode because wood is slightly acid, but when a metal fastener is
embedded in wet wood, conditions are created that can accelerate the corro-
sion of the metal. The corrosion products often result in slow deterioration of
the wood surrounding the metal. Corrosion of the fastener combined with
deterioration of the wood causes loss of strength of the joint and weakening
of the structural integrity of the assembly. The literature on wood-metal cor-
rosion has been reviewed carefully by Graham et al [ 1 ].
2
The causes and cures of wood weakened around corroding metal have been
1
Chemical engineer, Forest Products Laboratory, Forest Service, U.S. Department of Agri-
2
The italic numbers in brackets refer to the list of references appended to this paper.
culture, Madison, Wis. 53705.
981 982
DURABILITY OF BUILDING MATERIALS AND COMPONENTS
the subject of several research publications. Marian and Wissing [ 2-4 ] have
reported on the chemical analysis and the physical properties of wood in con-
tact with corroding steel. Farber [ 5 ] and Savard and Caumartin [ 6 ] have
shown the iron content of rail ties is high under tie plates and near rail
spikes; they indicate the loss of strength is due to the corroding steel. Pinion
[ 7 ] has reported the corrosion of fasteners in wooden vessels to be due to ox-
ygen concentration gradients between the exposed end of a fastener and the
portion embedded in the wet wood.
This report presents information on the theory of metal corrosion as it
relates to the corrosion products that initiate deterioration in the damp wood
that is in contact with the corroding metal fasteners [ 8 ]. In addition, new
data are presented on the corrosion of eleven fastener materials in water-
borne salt preservative-treated wood together with a brief discussion of the
corrosion of metals in contact with fire-retardant-treated wood products.
Corrosion of
Metal in Damp Wood
Isolated Metal
Fastener
For a single or an isolated metal fastener in damp wood, its corrosion can
be explained in terms of crevice corrosion theory. This type of corrosion oc-
curs in crevices such as a long tack welded, riveted, and bolted joints of tanks
and pipes. The following explanation for the corrosion of a steel nail in damp
wood is similar to that for crevice corrosion of a riveted section of steel in
aerated seawater given by Fontana and Greene [ 9 ]. Figure 1 shows the condi-
tions that are possible after corrosion has progressed.
Pinion [ 7 ] noted that the exposed end of a steel fastener in damp wood
quickly shows evidence of hydroxyl ion (OH-) formation. This indicates that
the exposed head of a nail, or of some other metal fastener, becomes the
cathode and the shank becomes the anode of a galvanic corrosion cell. The
chemical reaction at the cathode can be written
O
2
+ 2H
2
O + 4 e
4OH

The reaction at the anode for a steel nail can be written as follows:
Fe
Fe
++
+ 2 e
Ferrous ions (Fe
++
) liberated at the anode are oxidized readily to form fer-
ric ions (Fe
+ + +
) and can react to form either black iron tannate dyes or
rust. Iron ions are active catalysts and they promote chemical reactions that
cause a loss of strength in cellulose and wood [ 2 ].
Damp wood can be considered as an electrolyte or slightly acidic solution.
Soluble chlorides present in the bulk solution can migrate to a steel nail and BAKER ON CORROSION OF METAL IN WOOD PRODUCTS
983
result in accelerated corrosion of the nail and a weakening of the wood. As
the reactions at the anode and the cathode proceed, chloride ions (Cl

) and
hydroxyl ions (OH

) migrate from the bulk solution into the crevice between
the nail and the wood (Fig. 1). Iron ions formed at the anode in the crevice
react with hydroxyl ions in both the crevice and bulk solution to form insolu-
ble iron hydroxides. The formation of insoluble iron hydroxide in the crevice
leaves the solution acidic because of the relative decrease in concentration of
hydroxyl ions compared to that of the hydrogen ions (H
+
). The solution
within the crevice thus becomes acid due to high concentration of (H
+
).
FIG. 1 Schematic of conditions possible after crevice corrosion has progressed. 984
DURABILITY OF BUILDING MATERIALS AND COMPONENTS
These conditions accelerate the corrosion of the metal and the formation of
additional acid. Fontana and Greene [ 9 ] report that the fluid within crevices
exposed to neutral dilute sodium chloride solution has been observed to con-
rain 3 to 10 times as much chloride as the bulk solution and to possess a pH
of 2 to 3. The acid conditions developed along the nail shank can hydrolyze
the cellulose and weaken the wood.
The hydroxyl ions formed at the external cathode usually will not affect the
wood if exposure to free water will wash them away before any appreciable
concentration of hydroxide is reached.
Copper and its alloys undergo a different form of crevice corrosion than do
ferrous or aluminum-based metals when immersed in flowing water [ 10 ].
Copper enters the solution at both the exposed surface and the surface within
the crevice. At the exposed surface, the copper ions are washed away by the
water. This results in a higher concentration of copper ions in the crevice
relative to the concentration at the exposed end, thus establishing a differen-
tial metal-ion corrosion cell. The higher concentration of copper ions in the
crevice causes the copper metal in the crevice to become cathodic. The ex-
posed copper then becomes the anode. The predominant cathodic reaction is
the reduction of oxygen to form hydroxyl ions (alkaline conditions) which, in
time, will cause deterioration of the adjacent wood. This could explain the
condition of weakened wood found around isolated silicon bronze bolts in
minesweepers [ 11 ].
Crevice corrosion usually requires an incubation period to develop, but
once started it proceeds at an ever-increasing rate.
Stress corrosion cracking of tempered carbon steel nails (quenched and
tempered AISI 1035 carbon steel) has been observed when used in Douglas-
fir ( Pseudotsuga menziesii (Mirb.) Franco) and western larch ( Larix-occiden-
talis Nutt.) exposed to a warm humid environment over log heating chambers
in veneer mills [ 12 ]. A threshold level of susceptibility to stress corrosion
cracking was found after laboratory tests in Douglas-fir extract (pH 4.3) and
0.1 N acetic acid (pH 2.1). As a result nails with a hardness range of RHC 32
to 39, and an ultimate tensile strength range of 1000 to 1210 MPa, will be re-
quired for nails in a revised CSA standard 086. A hot dip galvanized finish
also will be required to resist occasional wetting.
Dissimilar Metals
Dissimilar metals in contact with one another in a corrosive environment
fo