• Corrosion phenomena in the petrochemical industry

Corrosion phenomena in the petrochemical industry

Oil & Gas / Petrochemical | Technical Article

The petrochemical industry faces a silent yet formidable adversary: corrosion. Critical equipment exposed to corrosive chemicals remains susceptible to this phenomenon, resulting in infrastructure deterioration, significant financial losses and, above all, safety risks. To combat this issue, petrochemical companies have developed sophisticated strategies centred on the use of protective coatings, corrosion-resistant alloys and state-of-the-art monitoring methods.

The complex world of petrochemicals

Petrochemistry encompasses a wide range of technologies that use oil or natural gas to produce a variety of chemical compounds. These processes involve complex chemical reactions and form the basis of many of the products we use every day, from solvents and medicines to plastics and fuels.

The challenges of petrochemical conversion

The products derived from crude oil distillation do not always meet market demands. This is where conversion processes come into play, performing essential molecular transformations. There are three types of cracking: thermal cracking, catalytic cracking and hydrocracking. While these processes produce the desired products, they also expose equipment to high temperatures and pressures, leading to corrosion and stress corrosion.

In oil refining facilities, chromium/molybdenum steels are widely used in the cracking section because of their creep resistance properties.

What is cracking?

In simple terms, cracking is the process by which crude oil is separated to extract by-products such as diesel, petrol, paraffin, etc. Each of these products vaporises at its own temperature, allowing optimal separation of these elements.

There are three cracking processes, detailed below:

Thermal cracking

Thermal cracking, the oldest process, was first applied to middle distillates (gas oils) and then to the light part of residues (vacuum distillates).

Through thermal decomposition, at temperatures close to 500°C, and under high pressure, these distillates are transformed into fuel. The reaction results in the formation of coke.

It was during the development of the first industrial thermal cracking facilities that the tubular furnace appeared – a true reactor for the unit. This was then widely adopted in all other refining facilities as an effective means of supplying heat to the products processed.

Currently, thermal cracking is carried out on the heavy part of the residues (vacuum residues), either moderately by visbreaking or severely by coking. It is also used with steam (steam cracking) in petrochemicals to produce olefins.

The widespread conversion of vacuum distillates and the reduction in the quantities of crude oil processed, led to the separation of all vacuum distillates. This has resulted in a correlative production of vacuum residue in excess of bitumen requirements. The highly viscous surpluses are diluted with diesel to obtain a blend viscosity in line with the specification required for heavy fuel oil.

Viscoreduction of the residue under vacuum is a moderate thermal cracking process that is increasingly used. This is because, in addition to producing less light distillates, it makes it possible to obtain a less viscous residue, therefore, reducing the quantity of dilution gas oil introduced into the heavy fuel oil and, consequently, the quantity of heavy fuel oil produced. In some cases, this quantity still exceeds market requirements.

In these cases, vacuum coking of the residue, which is a severe thermal cracking process, replaces visbreaking. The aim of this operation, which is usually discontinuous (delayed coking), is to produce coke, which is then burnt as is or gasified.

Catalytic cracking

Catalytic cracking of distillates under vacuum has long since replaced thermal cracking.

The catalyst, fluidised into fine particles, facilitates the reaction, which takes place at a pressure close to that of the atmosphere. It is continuously regenerated: the coke that settles on its surface is burnt as it goes along.

Industrial catalytic cracking plants are highly complex: in addition to the reactor and the regenerator, they include separation of the cracked products, a kind of synthetic crude oil, as well as energy recovery, since the regeneration temperature is close to 700°C.

Catalysts and reaction technology have evolved to increase both conversion and petrol production. Highly active catalysts are now molecular sieves, with contact times within a few seconds and petrol yields of around 50%. Facilities for catalytic cracking of atmospheric residue are beginning to be developed, but this product must first be purified to avoid poisoning the catalyst.

Hydrocracking and hydrodemetallisation

Catalytic cracking in the presence of hydrogen is carried out on vacuum distillates (hydrocracking) and atmospheric residues (hydrodemetallisation).

Hydrocracking produces an almost complete transformation into petrol, jet fuels and gas oils, and is carried out in a single or two stages, depending on whether the aim is to produce as much gas oil or petrol as possible. High hydrogen pressure (100 to 150 atmospheres), combined with moderate temperatures (350 °C to 450 °C), limits coke deposits on the catalyst.

The choice of chromium-molybdenum steel is appropriate in terms of creep resistance, given the high temperatures applied, ranging from 350 °C to 700 °C in the three conversion processes listed above. Unfortunately, it has its limitations regarding corrosion resistance and stress corrosion.

These steels lack corrosion resistance, necessitating the application of a protective cladding (corrosion protection) on the tubes to mitigate the corrosion rate.

Welding Alloys’ solutions for the petrochemical industry:

Welding Alloys offers a wide range of surface treatment solutions, including welding consumables, automatic welding machines and services, to combat corrosion.

Our product family comprises GAMMA nickel based alloys and COBALT based alloys, along with our range of TETRA austenitic cored wires. These materials effectively address the wear and tear induced by corrosion, whether through welding or cladding, commonly encountered in the petrochemical industry. They are specifically designed to increase wear resistance of pressure vessels and valve components, boasting distinctive characteristics tailored to these applications.

Welding alloys offer bismuth-free alloys (TETRA), as well as custom-made alloys for unique requirements.

One of the advantages of modern stainless steel flux cored wires, such as our TETRA range, is that the slag easily detaches, producing weld beads similar to those produced by the latest generation of coated electrodes.

The absence of bismuth in these wires prevents cracking at high temperatures; reactors often operate at temperatures of over 450°C.

Conventional rutile stainless steel flux cored wires generally contain added bismuth oxide (Bi2O3) to improve slag release. The welded deposit therefore contains traces of bismuth.

Most corrosion-resistant stainless steel welds are put into service as they are, without heat treatment. Such constructions are intended for applications where the operating temperature does not exceed 300°C. Under these conditions, the use of low-melting oxides to improve slag release is not a problem. However, many stainless steels are also used in applications where resistance to high temperatures is important. In such cases, constructions frequently have to operate at over 500°C, and in welded joints, bismuth and other low-melting elements can segregate at grain boundaries and cause cracking.

Wires with bismuth deliberately added are not recommended for applications above 500°C or where annealing treatment is carried out after welding. In such cases, we recommend the use of stainless steel cored wires containing no more than 0.002% Bi in the weld metal. These wires are specified in accordance with ASME II C SFA-5.22 and EN ISO 17633.

Welding Alloys has been producing wires meeting this criterion for many years and offers a wide range of bismuth-free cored wires. These products are suitable not only for high temperatures but also for welds undergoing stabilisation or solution heat treatment.

This range of cored wires provides the best compromise to fight against corrosion.

What are the differences between cored wire and solid wire?

A solid wire comes from a casting which is then drawn into welding wire. This leads to a batch size of several tons, characterised by the chemistry inherent in the produced batch.

A cored welding wire is the combination of one or a mix of several different powders enclosed in a metallic strip. This allows for the production of an infinite range of grades and chemistries in economical quantities, providing a distinctive advantage over solid wires.

In terms of weldability, flux cored wire offers much more versatility, such as welding with or without gas shielding, and welding in any position. We produce stainless steel flux cored wires that comply with standardised classifications and have a controlled and/or customised ferrite content, which is- of great benefit when it comes to combating corrosion.

Why use a flux cored wire instead of a solid wire?

A cored wire can be considered as an automatic coated electrode; a coated electrode consists of a metal core coated with a powder. A cored wire is made up of a strip that is formed into a U shape to incorporate a powder, then closed to form a tube. It is, therefore, a reverse-coated electrode that shares many similarities in terms of welding. These include the ability to produce a variety of alloys, the penetration profile, and the smoothness of the arc, among other characteristics.

Why Choose Welding Alloys?

There are a number of reasons why Welding Alloys should be chosen as a supplier of choice to combat wear effectively. These include:

Technical expertise: Welding Alloys is a world leader in the production of welding products dedicated primarily to repair and maintenance applications. Welding Alloys has extensive technical expertise and is able to provide high-quality solutions to a wide variety of welding and coating problems.

Innovation: Welding Alloys continually invests in research and development to improve the performance and durability of its wear-resistance solutions (hardfacing, corrosion-resistance, cladding, etc.).

Quality: Welding Alloys products are renowned for their quality. Welding Alloys uses carefully selected raw materials and follows rigorous production processes to guarantee high-quality end products.

Customer service: Welding Alloys places great importance on customer satisfaction. Welding Alloys offers quality customer service and is always ready to help customers solve their welding and coating problems.

Product portfolio: The most extensive range of welding alloys and wear-resistant coatings (corrosion, abrasion, impact, etc.) suitable for many industries. This means customers can source their repair and maintenance solutions from a single supplier, simplifying supplier management and logistics.

Are you facing problems with corrosion? We can help assess the situation with a no-obligation wear audit.

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