(WHITE RUST/WHITE STAIN)
Wet
storage stain, also known as white rust, is the voluminous white or
gray deposit formed by accelerated corrosion of the zinc coating when
closely-packed, newly-galvanized articles are stored or shipped under
damp and poorly ventilated conditions.
It
is found most often on stacked and bundled items, such as galvanized
sheets, plates, angles, bars, and pipe. Weathered zinc surfaces which
have already formed their normal protective layer of corrosion products
are seldom attacked.
Due
to their configuration, many products galvanized after fabrication are
less susceptible to wet storage staining than plain galvanized wire and
sheet. Wet storage stain results from the exposure conditions and is not
indicative of inferior or poor quality galvanizing.
The
bulky white or gray corrosion product associated with wet storage stain
should not be confused with the protective layer of zinc corrosion
products which form under normal atmospheric exposure of galvanized
coatings. Even though the corrosion products on fully exposed galvanized
surfaces may be white or light gray, they are not the product of wet
storage stain.
Their
color is solely a function of the environment and the zinc-iron alloy
content of the galvanized coating. When wet storage staining is found on
galvanized materials, it is not usually in sufficient quantity to be
detrimental to coating protection. Normally it disappears with
weathering. However, with ill advised transportation, handling and
storage methods, it can become serious.
While
the galvanizer has a responsibility to produce a galvanized coating of
good quality in conformance with governing specifications, the
galvanizer is no longer in control after the work leaves the plant. The
purchaser must ensure the proper handling and storage of material at the
construction site to ensure a stain-free product at the time of final
installation.
NORMAL GALVANIZED COATING CHEMISTRY
Before
considering how wet storage stain occurs on a galvanized part, it is
important to understand how freshly coated zinc surfaces interact with
the atmosphere. Once the basics of the zinc-atmosphere interactions are
understood, the formation and prevention of wet storage stain can be
explained.
Since
zinc is very reactive, any zinc surface in contact with the surrounding
air will quickly form a layer of zinc oxide. The formation of this
thin, hard, layer is the first step in the development of the protective
corrosion product film normally associated with galvanized coatings.
When
this surface has access to freely moving airin normal atmospheric
exposure, it reacts with rainfall or dew to form a porous, gelatinous
zinc hydroxide corrosion product. During drying, this product reacts
with carbon dioxide in the atmosphere and converts into a thin, compact
and tightly adherent layer of corrosion products consisting mainly of
basic zinc carbonate generally written as 2ZnCO3·3Zn(OH)2.
The
long life normally associated with galvanized coatings in atmospheric
service is entirely dependent upon the protection of the basic zinc
carbonate. Being relatively insoluble, the basic zinc carbonate layer is
weather-resistant and, once formed, minimizes further corrosion. After a
period of time, this whitish-gray powdery film tends to mask the
underlying zinc crystals on the surface of the galvanized coating.
The
degree of protection obtained in a given case varies with the nature of
the environment. The presence of chlorides and sulfur gases in the air,
for example, modify the composition of the carbonate layer and tend to
increase its solubility allowing rain and moisture to dissolve the
carbonate layer more rapidly. Thus the life of a galvanized coating is
generally shorter in marine
and
industrial atmospheres than it is in the cleaner air of a rural
environment. On a galvanized part, the basic chemical conversion on the
surface of zinc metal to zinc carbonate is the mechanism that is
interrupted by the formation of wet storage stain.
WET STORAGE STAIN CHEMISTRY
When
galvanized articles are closely packed, deprived of freely moving air
and exposed to moisture, an entirely different set of conditions arises
in the chain of zinc chemical changes. Without the free flow of carbon
dioxide over the galvanized surface, the surface retains water and forms
an “oxygen concentration cell.”
The
effect may be best explained in terms of the reactions which occur
under a drop of water flattened between two surfaces. The resultant
water film obviously has only a minimal surface exposed to air; meaning
that the zinc surface near the middle of the water drop or film gets a
different supply of oxygen than does the zinc surface at the periphery
of the water drop. This difference in turn sets up a difference in the
electrolytic potential of the zinc. The central area becomes anodic and
the edge area becomes cathodic, thus creating theOxygen concentration
cell (Figure 1).
The
attack on the zinc occurs at the anodic area. The corrosion products
are modifications of the nonprotective relatively soluble zinc
hydroxide. Since this hydroxide remains rather stable under these
conditions, zinc ions constantly leave the coating to be bound by water,
thus accelerating corrosion. The limitation of access to oxygen is also
a limitation of access to carbon
dioxide
and this hinders conversion of zinc hydroxide to the protective zinc
carbonate. The corrosion products in this case are not protective and
corrosion proceeds as long as the original conditions prevail. The
moisture necessary for the formation of wet storage stain may originate
in various ways. It may be present on the galvanized parts at the time
of stacking or packing, as a result of incomplete drying after
quenching. It may also be a result of direct exposure to rain or sea
water, or from condensation caused by atmospheric temperature changes.
Close packing can result in moisture being retained by capillary action
between the surfaces in contact because drying is delayed by the lack of
circulating air.
The
extent of the damage by wet storage stain depends on the duration of
exposure to retained moisture and the environment. The attack is
accelerated when the retained moisture contains chlorides from sea
water, sulfur compounds from industrial environments, or flux residues
from the galvanizing operations. Each of these contaminants heightens
the oxygen concentration cell’s effect by increasing the water’s
electrical conductivity. The important thing to realize, however, is
that severe damage can be done in a relatively short time by water
alone, without any other contributing factors. Because the corrosion
products of wet storage stain are voluminous, any attack may appear more
serious than it actually is. The volume change from zincmetal
to
zinc-oxide or zinc-hydroxide is 3 to 5 times greater. Medium or even
heavy layers of wet storage stain are very unsightly, but they represent
the loss of very little zinc from the base coating. The thicker zinc
coatings provided by after fabrication hot dip galvanizing usually
result in wet storage stain having little or no significant effect on
the durability and intended service life of the coating.
STORAGE AND TRANSPORTATION
Whenever
galvanized articles are packed closely together for appreciable periods
of time, adequate precautions should be taken against wet storage
stain. Wet storage stain can be minimized by maintaining a low humidity
environment around the material and by providing
adequate ventilation between the stacked pieces. Points to be observed are:
1. The galvanized ste
el, as produced, should be clean and free from flux residues.
2. The material should be stored under cover in dry, well ventilated conditions, with heating
facilities. It is very important to store the materials away from open doorways.
3. If outdoor stacking is unavoidable, the articles should be raised from the ground and
separated
with strip spacers to provide free access of air to all parts of the
surface. They also should be inclined in a manner which will give
maximum drainage. Do not store galvanized steel on wet soil or decaying
vegetation.
The
use of spacers is also recommended during any shipping if there is the
likelihood of condensation. For example, where material is chilled in
traveling over mountains and then exposed to warmer and more humid air
at lower levels, spacers are mandated. It is important that resinous
wood not be used for spacers or packing, since the resin itself can
start corrosion. Woods recommended for use in transporting and storing
galvanized products should be dry and untreated with preservatives or
fire retardant chemicals. Poplar, ash and spruce have been
used quite satisfactorily in contact with galvanized steel in storage and transport.
4. Small items that are quenched and stored in containers should be thoroughly dried before
packing. When the containers are sealed, the inclusion of a
desiccant is recommended.
5. Uncovered material should not be left standing at in-transit loading points where it may be
exposed to rain, mist, condensation, or snow.
6. When shipping overseas, galvanized steel should not be consigned as deck cargo or stowed in
parts
of the ship’s hold where contact with bilge water is likely. As noted
before, sea water is especially corrosive under conditions conducive to
wet storage stain. The high humidity at sea, particularly in the
tropics, makes the provision of dry, well ventilated facilities
particularly important.
SURFACE TREATMENTS TO INHIBIT CORROSION
Various
surface treatments are available to reduce the possibility of wet
storage stain. Conduit and hollow ware often are coated with a clear
film after galvanizing. Waxes and oils are commonly used on products
such as wire, sheet steel and fencing. For after fabrication hot dip
galvanizing, chromate dip treatments, or other proprietary solutions,
are sometimes utilized.
Most after fabrication hot dip galvanized products are shipped without any post treatment.
The
need for a surface treatment largely depends on the configuration of
the product and the anticipated storage conditions. Galvanized items
which are tightly stacked or nested are particularly vulnerable to wet
storage stain, especially if they are stored as unopened bundles for
more than a few weeks. On the other hand, fabricated assemblies, which
fully expose the galvanized surfaces, typically do not need a post
treatment. If the galvanized surface will be painted within six months, a
post treatment may interfere with paint adhesion. Communicating with
the galvanizer will establish how to prepare the surface for painting.
REMEDIAL ACTIONS
Wet storage staining of galvanized coatings is often superficial, despite the presence of a bulky
corrosion
product. In the vast majority of cases, wet storage stain does not
indicate serious degradation of the zinc coating, nor does it
necessarily imply any likely reduction in the expected life of the
product.
When
wet storage stain has occurred, the objects should be arranged so that
their surfaces dry rapidly. Once dry, the articles should be examined.
Where the surface staining is light and smooth, as seen on the guardrail
in Figure 8, or as judged by lightly rubbing the fingertips across the
surface, the stain will disappear gradually and blend with the
surrounding zinc surface as a result of normal weathering in service.
When the affected area will not be fully exposed in service, or when it
will be subject to a humid environment, wet storage stain should be
removed, even if it is superficial. This allows for the formation of the
protective zinc carbonate
film.
Medium to heavy build-up of white corrosion products, as shown on the structural tubing in
Figures
6 and 7, must be removed, otherwise the essential protective film of
basic zinc carbonates cannot form in affected areas. Deposits can be
removed by brushing with a stiff bristle (not wire) brush.
A
coating thickness check should be performed on the affected areas to
ensure that sufficient zinc coating remains after the removal of the wet
storage stain. In advanced stages of wet storage stain, the typical
white or gray corrosion product may become black in color. When this
occurs, a significant amount of coating has been lost to corrosion and
the service life is decreased.
In
extreme cases where heavy white deposit or red rust has formed as a
result of prolonged storage under poor conditions, as shown on the large
tubes in Figure 5, corrosion products must be removed and the damaged
area repaired as detailed in ASTM A780 Standard Practice for Repair of
Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings. Where the
affected area is extensive, or when the wet storage stain would impair
the use of the article for its intended service, regalvanizing may be
necessary.
