Aluminum Brightening

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Here is an article on Aluminum Brightning from www.pwnabbs.org

Aluminum Brightening

Aluminum is a metal that is relatively strong in comparison to its weight. This makes it an attractive metal for use in the construction of transportation equipment because a transporter of goods can move more products at a lower cost. It is normally used in the construction of the shell. Unfortunately, aluminum will oxidize and lose its luster. Normally oxidation on aluminum is white but when a container is transported down the highway the oxidation picks up dirt and pollutants that become imbedded within the oxidation causing it to take on the color of those pollutants. The color will vary depending on the environment that the aluminum is exposed to; turning it shades of brown, black, or red. This oxidation is difficult to remove using normal truckwash detergents. This is why after a typical wash job, using an alkaline soap and brushing; the surface may still remain dirty. In order to remove this oxidation a chemical abrasive is normally used such as an acidic or strong caustic solution. To use these chemicals effectively equipment, application, temperature, and safety must all be considered.

First and most important, always remember to keep safety in mind when working with acids these are aggressive cleaners that can cause bodily harm. Employees should be trained in proper use, safety precautions and treatment to neutralize over exposure. Protective gear should include, but not limited to, acid resistant protective wear such as rubber gloves, rubber boots, jacket, pants, and face shield.

Brightening removes an extremely fine layer of the aluminum surface exposing a new one. This is not flawless, a chemical process known a passivating occurs whereby small pits form. The degree of passivating will vary with the type of brightner, its strength, and dwell time. This pitting process causes the light to be reflected at different angles when striking these pits thereby changing it’s appearance. The change in appearance will move from a polished look to satin to a chalky one, as the pits become deeper and more abundant. Furthermore, these pits will cause the aluminum to get dirty faster. As the surface of a container has contaminates moving across it they will fall into the pits and become lodged.

Temperature has an effect on the cleaning ability of acid. Generally acids work best when the temperature is above 70 degrees. The warmer it is the more active the acid will be. Therefore it is best to heat the water if possible to maximize usage.

Most pressure washers are not designed for extreme chemical resistance. Therefore the use of up stream injection to apply the chemical is not an option. Meaning the chemical should not be pulled through the pump because it could cause extensive damage to the pump, unloader, coils, etc. Two other options are available known as down stream injection, after the pump, and the use of a chemical applicator. Typically, a down stream injector is coupled into the pressure line where the chemical will only flow through the pressure hose and the gun to the operator. This is accomplished by using a device to switch between low and high pressure at wand. The low pressure side of the switch will create a vacuum at the injector causing chemical to be drawn in, this is called a venturi draw effect. Whereas the high pressure side of the switch eliminates the vacuum and stops the flow of chemical into the pressure hose. Pump up sprayers and low volume pumps that are chemically resistant may also be used as a direct method to apply a brightner.

If the surface to be brightened has a fair amount of soil and grease on it, it is best to remove them prior to applying the brightner. This will reduce the usage and repeated applications required to achieve desired results. Furthermore, acid is a very poor cleaner for removing hydrocarbons (greases and oils). Using acids on the surface of aluminum must be done in a manner to prevent streaking. To get the best results work with small areas and apply the acid from the bottom up. Otherwise streaks will form as the chemical runs down the side of the container where no acid has been applied. It is good to follow this with a detergent that has a PH above 9 to neutralize the lower PH of the acid. The detergent is applied from the top down. The reason for using a neutralizer is to eliminate the possibility of any unwanted corrosion to the container after the acidizing process. It is not uncommon for the aluminum to be bond together other bare metal such as steel rivets and bolts. These other exposed metals will react differently with the acid and may cause black streaks to form below them if the acid is left on the surface too long. The dwell time for the acid should be from 30 to 60 seconds. Do not allow the acid to dry because once it dries on the surface it will lose all its cleaning ability. Furthermore, acid can etch glass causing a haze. If this happens a glass detching compound can be used to restore the surface.

Polished aluminum presents the most challenge to the contractor since the only way to remove oxidation is to remove the polish. The best possible solution is to do this with minimal effect to the shine. This is achieved by applying brightners at very low concentrations. Brightening should be postponed as long as possible.

Most all aluminum brightners have one of the following for a bases: Hydrofluoric acid (Amoniumbifloride – granule form of Hydrofluoric Acid), Sulfuric Acid, Phosphoric Acid, Hydrochloric Acid (Muratic), or Sodium Hydroxide (Caustic). Hydrofluoric acid used to be a very common acid used to brighten aluminum but do to the safety hazards involved with using it it’s not as common as it used to be. This acid will cause no irritation when applied to the skin instead it will cause discomfort after it reaches the bone and begins to react with the calcium. Due to this, most suppliers and end users have switched to one of the other brightner bases. The most aggressive acids are Sulfuric and Hydrofluoric, which will clean faster but will give the aluminum more of a chalky appearance. Phosphoric Acid being a less aggressive acid will leave aluminum with more of a satin finish. It is the acid of choice for use on polished aluminum surfaces. However, with repeated use, it too will eventually turn the surface of the aluminum chalky. Caustic and Hydrochloric Acid can actually burn the surface if left on too long. This burning will turn the aluminum a brown or black. If this happens just neutralize the surface and reapply the chemical to remove burn. Furthermore there is the possibility that hydrogen embrittlement may occur with the use of Hydrochloric Acid. A process where by the metal becomes brittle from the acid and it is weakened. It should be obvious that Hydrochloric should never be used to brighten any aircraft. Hydrochloric will also rust steel quite rapidly.

In conclusion, keep the following in mind when working with these brightners. The reactivity of the brightner will depend on type, concentration, dwell time, and temperature. Be aware of where the overspray will go, these chemicals can have undesirable effects on surrounding equipment. In general, keep up with past and current developments that effect your trade by reading trade publications as this one and joining trade organization such as PWNA (Power Washers of North America).

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Michael Hinderliter

Steamaway, Inc. Fleet Washing and Kitchen Exhaust Cleaning

www.steamaway.com, Phone:888-39-STEAM,

Certified in Kitchen Exhaust Cleaning and Environmental Cleaning.