FLASH RUST
PREVENTION AND
PROTECTION
WHAT IS FLASH RUST?
Flash Rust is defined as the rapid onset of corrosion that occurs on the surface of a metal
shortly after it is exposed to air and moisture. According to the Society for Protective Coatings
(SPCC), this type of corrosion generally occurs within minutes to a few hours after moisture
exposure. Flash Rust is primarily a concern in the blast cleaning industry.
Wet cleaning methods, such as wet abrasive blasting, can promote or accelerate the formation of Flash Rust. During wet blasting, high-pressure, high-velocity water impacts the metal’s surface in an attempt to remove unwanted material, such as old paints and contaminants.
However, wet blasting can also remove or damage the metal’s protective passive oxide layer.
This leaves the metal substrate unprotected and vulnerable to corrosion attack as it becomes
exposed to air and moisture. As a result, steel that is cleaned using wet blasting methods has
the potential to develop rust rapidly.
The rate of rusting is dependent on several factors, including:
◉◉ The purity of the water (i.e., the concentration of salts, chlorides and other corrosive ions)
◉◉ The dissolved oxygen content of the water
◉◉ The nature and concentration of contamination on the metal’s surface
◉◉ The humidity of the atmosphere
◉◉ The temperature of the environment, and
◉◉ The overall wet blasting drying time
While Flash Rust is prevalent during wet blasting, it can also occur during dry blasting or
any other type of surface preparation method. Like wet blasting, dry blasting can cause mechanical damage to the metal’s protective oxide layer. In high humidity environments, contaminants (such as salts) present on the metal substrate draw moisture from the air to the
surface. The combination of steel, oxygen and moisture from the humid environment results
in the formation of a corrosion cell.
2
Visible Flash Rust is categorized in terms of its severity. Common Flash Rust classifications
are:
LIGHT
MODERATE
HEAVY
The surface, when viewed
without magnification,
shows small amounts of
rust. The discoloration can
either be evenly distributed
or in patches. Generally,
light Flash Rust can be easily removed by lightly wiping
with a cloth.
Moderate flash rust is
characterized by a layer
of rust that is significant
enough to obscure the
original metal surface. It
is usually present as an
evenly distributed layer or
patches. It adheres to the
surface better than light
Flash Rust and leaves light
marks on a cloth when
wiped on the surface.
Heavy Flash Rust is defined
as rusting that hides the
original metal surface
completely. Like its light
and moderate counterparts,
heavy Flash Rust can appear
as either a uniform layer
or patches. This layer of
dark-brown rust adheres
loosely to the surface and
will easily mark objects
that are brushed against it.
Inspectors should expect to
see significant amounts of
color transfer when a clean
cloth is wiped on the surface.
3
METHODS OF FLASH RUST PREVENTION
Light Flash Rust on metallic surfaces generally fall within the tolerance levels of most coatings – coating manufacturers should, however, be consulted to determine the extent of Flash
Rust that a specific coating can tolerate for a given exposure environment.
Applying paints or primers to a moderately or heavily rusted surface can adversely affect
the adhesion of the coating to the metal. As such, appropriate measures should be taken to
prevent this type of corrosion when possible. Some of the most common techniques used to
avoid Flash Rust are:
1. MINIMIZE BLAST WATER IMPURITIES
During wet blasting procedures, the quality of the blast water is of utmost importance to
prevent Flash Rust after cleaning operations. Companies that use well water for their wet
blasting operations return shortly after cleaning only to find that the surface has rusted. This
rusting is due to the relatively high concentrations of harmful chlorides and other impurities
in the well water.
While the mechanisms by which chlorides promote corrosion are not entirely understood, it
is believed that chloride ions penetrate and degrade protective oxide films easier than other
ions do. The risk of flash corrosion increases as the chloride ion content increases. Chlorides
and other ions are also responsible for enhancing the conductivity of the water; thus facilitating electrochemical corrosion reactions.
Purified water on a clean metal surface will not cause corrosion; therefore, the aim is to minimize the presence of impurities in the blast water. Water filtered by distillation or reverse
osmosis can help prevent the probability of flash rust formation. At high levels of purification,
the use of filtered or distilled water can also reduce or eliminate the need for other protection
methods.
2. ENSURE THE METAL SUBSTRATE IS FREE FROM CONTAMINANTS
In addition to the contaminants found in the blast water, contaminants present on the surface
of the metal substrate can also contribute to the formation of Flash Rust. These contaminants,
which are often left on the surface after wet or dry blasting, can draw moisture from the humidity in the surrounding environment to create a corrosion cell.
Surface contaminants can originate from several sources. One of the most common causes of impurities on metallic surfaces is preexisting rust. Corrosion products present on the
metal’s surface can be spread across the surface during wet blasting operations, resulting in
4
widespread and evenly distributed Flash Rust over
the surface of the metal. Contaminants may also be
deposited on the surface by rain, airborne pollutants
or the steel forging process.
Salts and chlorides are among the surface contaminants that have the potential to cause the most damaging effects. In addition to drawing moisture from
the atmosphere, when mixed with the blast water,
these aggressive ions can erode the passive layer.
The resulting electrolyte solution then facilitates the
flow of electrons in the resulting corrosion cell.
Once blasting operations have taken place and the metal’s protective layer is damaged or
removed, these impurities are then free to attack the substrate unabated. As previously mentioned, if both the water and surface are pure (i.e., free of contaminants), then corrosion cannot occur. Several different cleaning agents can be mixed with the water during wet abrasive
blasting. These chemicals act as inhibitors or passivators, allowing the water to penetrate
into the substrate’s profile to flush salts and chlorides out of deep surface grooves. This
comprehensive cleaning method leaves the metal surface considerably cleaner and renders
it more inert.
Additionally, mechanical methods, such as hand/power tools and dry blasting, are especially
effective in removing salts. Since no water is involved, salts can be removed without having
to worry about them becoming dissolved in an electrolyte.
3. ADD A CORROSION INHIBITOR TO THE BLAST WATER
Corrosion inhibitors, as their name implies, are chemical compounds that inhibit or hinder
the formation of corrosion. During wet abrasive blasting, these inhibitors can be added to the
water prior to the commencement of blasting activities.
These corrosion inhibitors work in a variety of different ways. Some are specially formulated
to effectively remove contaminants from the metal’s surface, while others contain chemicals
that protect the surface by suppressing the electrochemical processes that cause oxidation.
Ultimately, the role of corrosion inhibitors is to slow down the corrosion process, thus, creating a “protection window” in which to apply the final coating.
5
COMMON INHIBITORS TO
PREVENT FLASH RUST
Listed below are some of the most common types of corrosion inhibitors used to prevent Flash
Rust during abrasive blasting:
PASSIVATORS
Passivators work by creating a thin protective film on the surface of the metal. The film provides a
type of protection known as barrier protection, where the film blocks air and moisture from coming
into contact with the bare metal substrate. Organic molecules, such as amines, have free electron
pairs that bond with the metal, inhibiting the adsorption of aggressive ions on the metal’s surface.
Passivators have, however, been known to negatively affect the adhesion and effectiveness of
some anticorrosive coatings and primers, particularly those containing zinc.
SALT REMOVERS
Salt removers, as their name implies, are used to remove salts ions from the surface of the
metal. These chemical compounds are formulated to dissolve salts, which are then washed
away during blasting. However, these solutions also dissolve the protective metal oxide layer, which allows them to interact directly with the substrate. As such, if they are not rinsed
off within the appropriate time, the acidic additives present in salt removers can eventually
result in osmotic blistering of the surface.
SURFACTANTS
Surfactants work by decreasing the surface tension of the water; thus, increasing the wetness
of the liquid. This allows the water to penetrate deeper into the steel profile to dissolve and
wash away salt deposits. Surfactants can also assist in evaporation and leave no residual
contaminants on the substrate.
Although corrosion inhibitors are effective at preventing Flash Rust, they can also have some unwanted side effects. Firstly, some inhibitors may interfere with the ability of coatings to adhere to
the metal’s surface. In these cases, the coating manufacturer should be consulted regarding the
interaction of their product with various types of corrosion inhibitor compounds.
Also, some inhibitors may contain carcinogens and other toxic compounds, making them a
risk to public and environmental health and safety. Adequate caution should be exercised
when selecting and applying specific corrosion inhibitors.
6
INSPECTION
While inspections cannot avert the onset of flash rust, it can help prevent or mitigate some
of its adverse effects. There are a number of flash rust evaluation techniques available,
all of which are based on the ease of which the rust can be removed. The most common
Flash Rust inspection techniques are the brush cloth test, the ten tape test and the tape
transmittance test.
BRUSH CLOTH TEST
The brush cloth test is used to measure the degree to which flash
rust can be removed from the surface by “lightly wiping” it with a
cloth. The criteria for this inspection technique are outlined in SSPC
SP-12/NACE No 5. This method is a more detailed version of the
traditional “white cloth test” and allows for greater consistency of
observations among different inspectors.
This method involves lightly wiping the rusted metal surface using a cloth wrapped around a
paint brush. Both the type of cloth and brush are standardized to minimize subjectivity among
inspecting individuals. The amount of rust transferred to the cloth is then evaluated against
reference photos provided in the standard.
TEN TAPE TEST
The ten tape test involves the sequential application of different
strips of tape on the same location of a rusted metal surface.
First, the spot to be tested is determined and a piece of tape of at
least 2 inches in length (as specified in ASTM D3359) is attached
and rubbed thoroughly onto the surface. The tape is then peeled
off and placed on a white reference surface.
The tape application process is repeated nine more times on exactly the same spot using
a new piece of tape each time. The tenth piece of tape is observed and assessed using the
Hempel rating scheme to determine the degree of flash rust on the surface. This test, however, is tedious to perform and does not provide a quantitative measure of rusting.
7
TAPE TRANSMITTANCE TEST
In an attempt to improve on the shortcomings of the ten tape
test, the U.S. Navy developed the tape transmittance test. This
test is performed by obtaining the first tape pulled off of a flash
rusted surface and analyzing its light transmittance properties,
i.e., the amount of light allowed to pass through the tape sample.
This test procedure gives more quantitative results than the ten
tape test.
The test sample is prepared by applying a 4-5 inch length of dust
tape to the rusted surface in accordance with ISO 8502-3. The
tape is rubbed in thoroughly, removed, and then placed on a clear microscope slide. The
amount of light passing through the tape and microscope slide assembly is measured using a
transmittance meter. Several readings are taken, and the difference in the average transmittance of the test sample and control sample are observed.
CONCLUSION
Although Flash Rust occurs relatively quickly, it can be easily prevented by taking
the necessary precautions prior to wet or dry abrasive cleaning operations. Flash
rust prevention techniques are aimed primarily at reducing or eliminating the
presence of contaminants in the water used for abrasive blast cleaning and the
metal’s surface; both of which are critical elements in this type of corrosion. If
corrosion inhibitors are used, environmental conditions and blasting techniques
should be carefully analyzed to ensure that the selected product performs as
intended.
8