Corrosion problems and solutions of carbon steel flange and stainless steel flange connection

The corrosion problems of metal materials in the petrochemical industry are summarized. Combined with the petrochemical projects at home and abroad in recent years, the different preventive measures for metal material corrosion are analyzed and discussed. The corrosion problems and solutions of carbon steel flange and stainless steel flange connection are emphatically discussed.

As an important pressure-bearing equipment for petrochemical production, petrochemical equipment plays a decisive role in the safe and stable operation of the entire petrochemical plant system. There are many kinds of petrochemical equipment, including reactors, cold exchange equipment, separation towers, pressure storage tanks, transmission pumps, and connected pressure pipelines. The operating conditions of petrochemical engineering devices are complex and changeable. The pressure-bearing equipment is often affected by various acid-base corrosive solutions during operation. In addition, the high temperature and high pressure environment will aggravate its corrosion, and its performance and life will be damaged. Serious corrosion may lead to major safety accidents and cause huge losses to life and property.
Flange connection is the most common between pressure equipment and pressure pipeline and between pressure pipeline components. In petrochemical plants, the matching connection of carbon steel flange and stainless steel flange is common, leading to the contact corrosion of dissimilar metals between carbon steel and stainless steel flange. In this paper, the corrosion problem is analyzed in detail, and the solutions of different projects are discussed to help and inspire colleagues engaged in the corrosion protection of petrochemical metal materials.

1. Metal corrosion

1.1 Definition of metal corrosion

Metal corrosion is the destruction or deterioration of metal materials caused by chemical or electrochemical action in the external environment (medium).

1.2 Mechanism of metal corrosion

According to the process of metal corrosion, corrosion is generally divided into two categories: chemical and electrochemical. Chemical corrosion is the destruction caused by the direct chemical reaction with the environmental medium; the most common is the oxidation reaction of metal materials; electrochemical corrosion is the galvanic reaction of impure metal or alloy in an electrolyte solution, and the more active metal is oxidized and consumed by the loss of electrons.

1.3 Classification of metal corrosion

The mechanism of metal corrosion is more complex, and the corrosion phenomenon is also diverse. In general, it is classified according to three aspects: corrosion mechanism, corrosion morphology, and corrosion environment, as shown in Table 1.

Table.1 Classification of metal corrosion

Based on	Category
Corrosion mechanism	Chemical corrosion: During the corrosion process, the transfer of electrons is carried out between the metal and the oxidant, and the corrosion does not produce current. Electrochemical corrosion: Current is generated during the corrosion process, and the electron flow inside the metal and the ions in the medium form a current loop. Physical corrosion: metal damage caused by simple physical dissolution is relatively rare. Uniform ( comprehensive ): film corrosion, no film corrosion.
Corrosion form flashing	Local corrosion morphology: pitting corrosion, crevice corrosion, delamination corrosion, intergranular corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction vibration corrosion, hydrogen embrittlement, hydrogen blistering and hydrogen corrosion.
Corrosive environment	Acid corrosion, alkali corrosion, salt corrosion, atmospheric corrosion, soil corrosion, seawater corrosion, freshwater corrosion, etc.

1.4 Hazards of metal corrosion

Corrosion hazards are common in all walks of life in the national economy, such as petroleum, chemical industry, energy, transportation, machinery, food, medicine, etc. Statistics show that the annual economic losses caused by corrosion account for about 3%-4% of the total national economy. The direct economic losses caused by metal corrosion worldwide yearly are about 700 billion-1 trillion U.S. dollars; every time the second hand on the watch turns a circle and a half, 1t of steel in the world is corroded into rust.
It can be seen from the above data that the consumption of natural resources and materials by corrosion is amazing. April 24, 2017, is the 9th ‘World Corrosion Day.’ Han Enhou, Chairman of the World Corrosion Organization and Researcher of the Institute of Metal Research, Chinese Academy of Sciences, said in an interview with the central media: Corrosion is a common problem faced by all humanity, involving the consumption of resources, affecting the environment and human health. The total cost of corrosion in China is about 5% of the gross domestic product (GDP), and the corrosion cost is greater than all natural disaster losses. Compared with natural disasters such as earthquakes and tsunamis, the destructive power of corrosion is also extremely strong. At the same time, because the occurrence of corrosion is silent and long, it is easy to be ignored.

1.5 Metal Corrosion Protection

Given the corrosion types of metal materials, targeted anti-corrosion measures need to be taken. The main protective measures taken in the petrochemical field are shown in Table 2.

Table.2 Protective measures against metal corrosion

Corrosion prevention	Remarks
Reasonable selection of metal materials	It is necessary to fully consider the corrosive environment in which the material is located and not blindly pursue low-cost materials
Isolation and protection of metal materials	Take painting and electroplating measures to isolate metal materials from corrosive media to slow down corrosion
Electrochemical protection	Eliminate the Primary battery reaction that causes electrochemical corrosion of metal by means of anodic protection or cathodic protection, so that metal can be protected
Strengthen construction and production management	Eliminate potential corrosion hazards of metal materials caused by construction, such as welding residual stress, local vibration, etc; Eliminate the misuse of metal materials and parts during installation, construction, and production of devices

2. Corrosion problem of flange connection between carbon steel flange and stainless steel flange

2.1 Corrosion mechanism of carbon steel flange and stainless steel flange connection

Carbon steel and stainless steel flange connection has the corrosion problem of different metal contact, mainly has the following two kinds of corrosion phenomenon:

• (1) Carburizing: Carbon in carbon steel migrates and forms chromium carbide with chromium in stainless steel. As a result, a ‘chromium-depleted zone’ is formed locally in the stainless steel material. Under the action of the corrosive medium, the chromium-depleted zone loses its corrosion resistance, as shown in Fig.1.
• (2) Electrochemical corrosion: The electrode potential of stainless steel is higher than that of ordinary carbon steel due to the alloy elements such as chromium and nickel. In a humid environment, the galvanic reaction occurs between the two, and the iron in the carbon steel loses electrons and becomes iron ions. Corrosion, while iron ions contaminate the stainless steel material in contact with it, see Figure 2.

Fig.1 Schematic diagram of the chromium-depleted zone at the grain boundary of stainless steel

Fig.2 Electron chemical etching mechanism
The two kinds of corrosion often occur simultaneously and affect each other. If you are in the acidic and humid atmosphere or electrolyte solution environment of a petrochemical plant for a long time, the corrosion will be further aggravated.

2.2 Carbon steel and stainless steel flange connection applications

In petrochemical plants, due to the influence of process conditions, equipment, and instruments, the matching connection of carbon steel flange and stainless steel flange is more common, mainly in the following occasions:

• (1) Flange connection of pipeline grade boundary of different materials;
• (2) Flange connection between stainless steel equipment nozzle and carbon steel pipe;
• (3) Flange connection of stainless steel instrument on carbon steel pipeline;
• (4) Flange connection of stainless steel special parts on carbon steel pipeline.

2.3 Corrosion hazard of carbon steel flange and stainless steel flange connection

There are two main corrosion phenomena in the connection of stainless steel flange and carbon steel flange: carburizing and electrochemical corrosion. Taking the austenitic stainless steel material as an example, when the carburizing phenomenon occurs, the excess carbon diffuses to the austenite grain boundary. It combines with chromium near the grain boundary to form chromium carbide. When the mass fraction of chromium near the grain boundary is less than 12%, the so-called “chromium-depleted zone” is formed, resulting in intergranular corrosion, destroying the bonding force between the grains inside the austenitic stainless steel, which greatly reduces the mechanical strength of the metal material. More tricky is that corrosion occurs between the grain boundaries inside the material, and the surface of the metal material still maintains a certain metallic luster, so the problem is difficult to be found.
The electrochemical corrosion hazards of stainless steel and carbon steel flange connections should also be paid attention to. The passivation films of two different metal materials have different properties and stability. After contact with the electrolyte solution environment, the destruction of the passivation film will be accelerated due to the potential difference, which will further aggravate the corrosion. The rust on the metal surface affects the appearance and further threatens the long-term safe and stable operation of petrochemical equipment.

2.4 Anti-corrosion measures of carbon steel and stainless steel flange connection

Given the corrosion problems in the connection of stainless steel and carbon steel flanges, targeted measures can be taken to slow down the corrosion process to effectively protect metal materials and prolong their service life. In the design, construction, and actual operation and maintenance of petrochemical projects, the following three anti-corrosion measures are mainly adopted to deal with the corrosion problems of dissimilar metal flange connections:

• (1) The metal surface is covered with a protective layer, such as paint, grease, etc., or electroplated zinc, chromium, and other easily oxidized metals, forming a dense oxide film as a protective layer;
• (2) The carbon steel flange system welds the zinc block. When the corrosion occurs, the active metal zinc replaces the iron element in the carbon steel to lose electrons and is corroded so that the carbon steel material is protected, that is, the sacrificial anode cathodic protection;
• (3) Insulation gasket is selected at the sealing surface of the flange connection, a fastening bolt and insulation sleeve are added, and an insulation washer is added at the contact between the nut and flange.

3. Some thoughts on the corrosion problem of carbon steel and stainless steel flange connection

3.1 External environmental factors and effects of dissimilar metal flange connection corrosion phenomenon

Carburization occurs when carbon steel and stainless steel materials are in direct contact. In the field of petrochemical engineering, pressure equipment, pipe fittings, and instrument components of different metal materials need to be stored separately and not directly contacted; stainless steel pipes shall not be placed directly on structural carbon steel beams and isolation pads (rubber pads or stainless steel sleeves) shall be added. Direct contact with dissimilar materials is a basic principle generally recognized and followed in engineering design, construction and installation, and quality acceptance. There is a view that the pickling and passivation of stainless steel flanges to form a dense passivation film on the surface can effectively block the corrosion of external media, and the process is simple, just soaking at room temperature. The passivation treatment of stainless steel surfaces can play a protective role, but it cannot be done once and for all. For petrochemical plants with harsh and complex external conditions (such as pressure equipment and pipelines that operate for a long time under high temperatures and high pressure), the long-term effect of an acidic corrosive environment and the external force intervention of operation and maintenance will destroy the passivation protective film on the surface of stainless steel. With the increase in ambient temperature, the carburizing rate will also accelerate.
The occurrence of electrochemical corrosion requires the following basic conditions: impure metal materials have electrode potential differences and are in an electrolyte solution environment. It is generally believed that the higher the temperature of the electrolyte solution, the faster the corrosion rate; the smaller the pH of the electrolyte solution, the faster the corrosion rate. The corrosion rate increases with the increase of salt concentration in the electrolyte solution. When the concentration further increases, the corrosion rate gradually decreases (the solubility of oxygen in the solution gradually decreases with the increase of salt concentration, and the depolarization effect decreases).

3.2 Balance between necessity and economy of corrosion protection for dissimilar metal flange connection

The corrosion problem of dissimilar metal flange connections needs attention and targeted protective measures should be taken. The author has combed the corrosion protection measures of petrochemical projects at home and abroad that have participated in the past ten years, as shown in Table 3.
From Table 3, it can be seen that the metal surface covering the protective layer is a protective measure generally adopted by petrochemical engineering plants, and cathodic protection measures are not widely used, only for areas marked as harsh corrosion environments; overseas projects additionally consider the use of insulation gaskets and bolt insulation sleeves and pay more attention to the corrosion of dissimilar flange connections than domestic petrochemical projects. In recent years, petrochemical enterprises have attached great importance to safety production and environmental protection but also pay more attention to the problem of metal corrosion and strive to balance safety production and economic benefits.

3.3 Precautions for selection of insulation gaskets and bolt sleeves for dissimilar metal flange connections

Given the corrosion protection of stainless steel flange and carbon steel flange connection, insulation gaskets, bolt sleeves, and insulation washers have not been widely used in petrochemical projects due to their limitations. First, the selection of gaskets is based on the conditions of medium, temperature, and pressure. Only when the above conditions are satisfied can the insulation be considered; secondly, after the insulation of dissimilar metal flanges, it is necessary to consider the separate electrostatic grounding on both sides of the flange for the pipeline transporting combustible fluid medium, which increases the grounding cost. Thirdly, the price of flange insulation gaskets for high-temperature, high-pressure, and corrosive medium pipelines is relatively high. Using insulation sleeves and insulation washers will also increase the project cost.

4. Conclusion 

In petrochemical plants’ design, manufacture, construction, and production operation stages, professionals must attach great importance to the corrosion of carbon steel and stainless steel flange connections in pressure vessels and pressure piping systems. Under the premise of fully ensuring the safe operation of the system, combined with the actual situation and process characteristics of the device, effective protective measures are taken economically and reasonably; during regular maintenance and overhaul, corrosion detection is carried out in a targeted manner, and metal material components with serious corrosion are replaced in time. The causes of corrosion are analyzed, and corresponding solutions are proposed.
Author: Xu Qingdong