How To Prevent Rust on The Dryer Cylinder

Types and mechanisms of dryer cylinder surface corrosion

There are two types of corrosion for dryer cylinders, namely electrochemical corrosion and stress corrosion.

In the atmospheric environment, water steam is prone to form a water film on the cylinder surface. O2, CO2, NO2, SO2, and salts in the air dissolve in the water film, forming an electrolyte liquid film on the cylinder surface. The material of the dryer is iron carbon alloy, which forms micro batteries under the action of electrolyte liquid film, resulting in electrochemical corrosion.

When the dryer is in a steam state, the cylinder body is subjected to internal pressure and thermal expansion, causing tensile stress on the cylinder surface. The tensile stress can damage the passivation film on the cylinder surface, causing corrosion on the cylinder surface, which is called stress corrosion. Therefore, the basic measures for rust prevention on the cylinder surface are to reduce the chemical activity of metallic iron and prevent the formation of water film.

Rust Prevention of Dryer Cylinder

After the dryer cylinder enters the paper mill, the surface of the cylinder should be checked for rust first. If there is light yellow rust, mechanical oil can be used to wipe it. If the rust turns black into flakes, M8 grit grinding paste can be used to wipe it, which generally does not damage the cylinder surface. If the rust is deep but does not affect the substrate, chemical rust removal method can be used. First, remove the local grease from the corroded area, then repeatedly wipe with a 5% concentration hydrochloric acid or sulfuric acid solution. After wiping, immediately clean with clean water and neutralize with a 1% sodium carbonate solution to prevent further corrosion of the cylinder surface by residual acid.

To prevent electrochemical corrosion during the initial use of the dryer, paper mills generally use animal, vegetable or mechanical oils to passivate and maintain the cylinder surface under steam flow, and the effect is satisfactory. The anti-corrosion mechanism is essentially the corrosion inhibition effect of organic polar groups on metals, which is similar to the corrosion inhibition effect of oil soluble corrosion inhibitors on oxidized petroleum lipids. After maintenance, the surface of the cylinder is glossy black and shiny, and the metal is passivated. The operator calls it a "cooked cylinder". In my opinion, it is better to use a saturated sodium nitrite aqueous solution for maintenance, which has a short operation time, fast drying speed, no need to clean, and is easy to use. However, sodium nitrite has carcinogenic effects and can synthesize carcinogenic substances nitrosamines and nitrosamines in the stomach after ingestion. Measures should be taken during operation to prevent ingestion into the body.

During the production of paper machines, the dryer cylinder works in a complex corrosive environment, and severe corrosion often occurs at both ends of the cylinder surface, while the effective use surface covered by the paper is not easily rusted. Research has shown that steel and cast iron are prone to forming dense corrosion product films during the wet dry alternation process, which passivate the metal surface and prevent the corrosion process. The dry corrosion product film is the inner passivation film generated during the wet dry alternation process. In continuous use, the outer layer rust does not occur due to the operation of paper wrapping. However, when the dryer is not in use (long-term shutdown), the cylinder surface will still adsorb water steam and produce rust. Therefore, when the dryer cylinder is shut down for a long time, it is necessary to apply oil for rust prevention.

The passivation film generated on the surface of the dryer cylinder during use has a corrosion-resistant effect. However, the non paper covered parts on both sides are often adhered by fillers, adhesives, fibers, etc. in the pulp. Under wet, hot and acidic conditions, only Fe2+ions dissolve, while amorphous hydroxide iron oxide and u-FeOOH, which do not have a surface passivation effect, are generated and precipitated. Although Fe2+combines with OH ions, oxidizes, and dehydrates to form Fe2O3 The oxide film is easily damaged by surface tensile stress, causing the metal grain boundaries to be in an active state and resulting in intergranular corrosion. Under stress, it develops into grain boundary cracks. Due to the ion enrichment at the crack front and the invasion of corrosive media, the cracks continue to propagate until small pieces fall off, forming corrosion pits. Corrosion pits make the surface rough and increase electrochemical corrosion. In severe cases, corrosion cracks will develop deeper into the cylinder surface under stress, reducing the strength of the cylinder surface and extending to the effective use surface, making the dryer unusable. Therefore, when using a dryer, especially one with a idler roller, a moveable doctor blade should be equipped, and the pressure of the doctor blade should be adjusted at any time to remove residual fibers from the cylinder surface, keep the entire cylinder surface clean, prevent accelerated corrosion, and improve the service life of the dryer.