I. CORROSION BACKGROUND IN PAPER MILLS

Paper is an essential material in modern society for both daily life and production. It is not only a cultural necessity but also a vital industrial material. The paper industry is a significant sector of the national economy and is a traditional manufacturing industry.

In the paper industry in China, many mills use the alkali method for pulping, with key chemical raw materials including caustic soda, liquid chlorine, and aluminum sulfate. The management of equipment in paper mills is closely tied to corrosion prevention, which is a critical task.

As is well known, corrosion is a major issue in industrial production. In domestic industries, the economic losses caused by equipment corrosion have exceeded 10 billion yuan. In the chemical industry, it has become a critical problem. The consequences of equipment corrosion range from minor issues like leaks, environmental pollution, energy waste, quality degradation, and increased costs, to severe impacts such as personal safety risks, production shutdowns, and reduced profits.

Ensuring that manufacturing equipment is protected from corrosion has become a top priority.

I. Reference Standards

Current national standards, relevant regulations, and mandatory standard provisions;

"Code for Construction and Acceptance of Building Anti-corrosion Engineering" (GB50212-2002)

"Standard for Corrosion and Rust Removal Levels of Steel Surface Before Coating" (GB8923-1988)

"Safety Regulations for Coating Operations and Safety Management Rules" (GB6514-1995)

"Quality Requirements for Anti-corrosion Coatings" (GB6514-1991)

"Safety Regulations for Coating Operations, Paint Process Safety, and Ventilation Purification" (DJ/T6931-1999)

"Code for Construction and Acceptance of Industrial Equipment and Pipeline Anti-corrosion Engineering" (HGJ229-91)

"Safety Regulations for Coating Operations and Safety of Surface Preparation Process" (GB7692-87)

"Noise Limits at Construction Sites" (GB12523-90)

ISO9001 Quality Management System Documents

ISO14001 Environmental Management System Documents

GB/T28001 Occupational Health and Safety Management System Documents

I. Design Basis

Environmental Conditions: The corrosion level of atmospheric environments on steel structures can be determined according to Table 1.

Corrosion Factor Analysis of Pulp Tanks

The pulp tank contains a mixture of pulp, acid, alkali, and bleach. The temperature usually ranges from 70 to 80 degrees Celsius, which accelerates the corrosion of the tank surface by the acidic and alkaline media. Based on this corrosion analysis, the anticorrosion requirements for pulp tanks should include: acid resistance, alkali resistance, good thermal stability, and resistance to impact, wear, and seepage.

Principles for Selecting Heavy-duty Anticorrosion Coatings for Pulp Tanks

1. Use epoxy floor sealing primer: This coating has strong permeability and excellent sealing performance, providing excellent adhesion to the substrate, alkali resistance, and waterproof properties, and is compatible with intermediate and topcoat layers.

2. Use epoxy mortar for acid and alkali-resistant intermediate coatings: After application, this coating has a certain thickness and provides excellent abrasion resistance, compressive strength, impact resistance, anti-slip, dustproof, alkali resistance, and resistance to other chemical media corrosion.

3. Use acid- and alkali-resistant epoxy glass flake heavy-duty anticorrosion coatings:

1) Excellent shielding effect and anticorrosion performance. The epoxy resin with excellent anticorrosion properties is used as the main film-forming substance, with glass flakes as the aggregate. The cured coating forms a "fish scale" shield layer and a high molecular network structure anticorrosion layer, which provides excellent corrosion resistance, water permeability resistance, gas permeability, acid resistance, alkali resistance, salt resistance, oil resistance, solvent resistance, and impact resistance.

2) Thick film, low pollution, saving on the number of construction layers, reducing project time, and achieving economic benefits. Due to the high solid content, there is less solvent volatilization, which reduces environmental pollution.

3) Convenient to use, cures at room temperature, with a simple construction process, not limited by environmental conditions, and can be used as needed with standard construction methods.

Table 1: Use of Graphene Paint Technology in Pulp Tanks

I. Corrosion of Pulping Equipment

In the pulping and papermaking process, equipment often comes into contact with corrosive media such as acids, alkalis, salts, corrosive gases, or microorganisms that have corrosive effects. As a result, the equipment is damaged by corrosion, leading to many adverse impacts on production, such as increased raw material consumption, worsened production environment, etc. In severe cases, equipment may become unusable before reaching its expected service life and must be replaced early. This greatly increases the costs of the papermaking industry and seriously affects the economic benefits of enterprises. Therefore, studying the causes of corrosion during production and seeking effective solutions, as well as implementing anti-corrosion measures, are of significant importance in the pulping and papermaking industry.

The main causes of corrosion in the papermaking industry are divided into two categories: chemical causes and microbial causes.

1) Corrosion caused by chemical factors: The water used in papermaking often contains various residual chemical additives, which can be considered as electrolyte solutions. The metal equipment surfaces are corroded due to electrochemical reactions.

2) Corrosion caused by microorganisms: Microorganisms can survive in papermaking raw materials and the production environment. In the papermaking production system, common microorganisms that can cause corrosion to equipment include sulfate-reducing bacteria, filamentous iron bacteria, and sulfur bacteria. These bacteria produce metabolites containing organic and inorganic acids, which promote the deposition of fibers, fillers, or inorganic salts on the equipment surface. The metal surface beneath the deposits becomes a cathode due to oxygen deficiency, forming a concentration cell, initiating the corrosion process.

Table 1: Pulping Equipment Using Graphene Paint Technology

II. Anti-Corrosion Plan for Papermaking Machines

The equipment process in a papermaking machine production line is long, with complex specifications and multiple systems. In this process, any malfunction of a single machine or component can lead to a complete production stoppage, causing economic losses for the enterprise. Therefore, equipment maintenance is a fundamental condition for ensuring the normal operation of a papermaking factory, and it is a key focus in equipment maintenance management.

The main causes of papermaking machine equipment failures are:

1) Wear: The main causes of wear include long-term contact with copper mesh, paper pulp, felts, or components and equipment that are subject to wear, such as pulpers, pulp pumps, vacuum box panels, sand removers, calender rolls, and press rolls. Another cause is frequent movement between components, such as steam inlet heads, vacuum press rolls, doctor blades, cylinders, and vacuum pumps.

2) Lubrication: Mechanical rotation and bearings are directly related. Papermaking machines, which mainly use rotating rolls, have numerous types and quantities of rolls, and hundreds of different bearing models. Therefore, lubrication management is extremely necessary. Particularly, due to the effects of water, heavy loads, and high temperatures, lubricants are prone to evaporation, loss, and aging, which damages the lubrication effect, leading to bearing damage and roll neck wear. Thus, regular lubrication checks are critical.

3) Fatigue: Heavy loads and continuous operation can cause fatigue damage in machinery. In papermaking machines, due to mechanical structure constraints and operating principles, certain parts cannot have excessively high design strengths. To avoid damage, maintenance or technical improvements can be employed as solutions.

I. Boiler Steel Structure Corrosion Mechanism Analysis

Boilers are essential equipment in papermaking plants. Corrosion is a problem that exists during both the operation and shutdown periods of boilers. Because the corrosion process is slow and not significant in the initial stages, it does not directly threaten the safety of the boiler and is often overlooked. However, once corrosion occurs, it can directly affect the safe operation of the boiler, and in severe cases, it may lead to forced shutdowns, causing significant economic losses.

During the shutdown period, various types of corrosion may occur in the boiler, and the necessary conditions for corrosion are water and oxygen. After the boiler is shut down, large amounts of external air enter the furnace system, and the water film attached to the metal surface of the furnace body combines with the air to form oxygen solution, reaching a saturated state and causing dissolved oxygen corrosion. The corrosion rate in this case is ten times higher than during operation. In addition, the remaining scale intensifies the corrosion. In a boiler filled with water, the boiler water, as a polar electrolyte, causes iron atoms to turn into positively charged iron ions, which is essentially an electrochemical corrosion process.

The gradual loss of iron atoms leads to the formation of increasing numbers of pits, thereby affecting the overall structural stability of the boiler. Corrosion can occur at any part of the boiler, and its distribution is irregular. The large amount of products generated during corrosion accumulates on the boiler surface, forming visible corrosion products. Depending on the oxygen supply conditions, these products include FeO, Fe2O3, and Fe3O4.

Table 1: Application of Graphene Coating Technology to Boiler Steel Structure