Weld Cladding : Frequently Asked Questions

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Can weld clad steel components be welded together...?
The aim of welding clad steel is to maintain a continuous fully corrosion resistant layer across the joint, to produce a fully corrosion resistant system.

Good cleanliness and dryness is essential to prevent contamination of the deposit with sulphur or hydrogen which could lead to cracking.
How is weld overlay cladding integrity tested and assured...?
There are a range of available specifications which can be used to test and ensure the integrity of weld overlay cladding. Most specifications require dye penetrant inspection of the clad surface to detect any cracks - e.g. API 6A.

If cracks are detected some specifications require a ferroxyl test of the crack to detect whether it reaches down to the underlying steel - this is only applicable to a single layer product.

Ultrasonic inspection may also be required - e.g. to ASTM A578, and there is now a complete specification for Corrosion resistant alloy clad and lined steel pipe - API5LD from the American Petroleum Institute.

find out more on the IODS QA and NDT pages.
What are the benefits of using corrosion resistant alloys...?
Today the decision to utilise corrosion resistant alloys should be made at the pre-engineering stage and full advantage taken of the higher allowable stress levels of downhole tubulars and of weight saving in platform and support structure design, utilising Corrosion resistant alloys for process and fire-fighting piping applications. Platform space can also be saved by removing the need for inhibition systems.

Metallurgy and production technology have been developed and established to allow the use of corrosion resistant alloys for deep drilling to meet very severe corrosive conditions - such as hydrogen sulphide concentrations up to 35% and carbon dioxide concentrations in the 3-10% range, in pressures approaching 22,000psi (150 MPa) and temperatures reaching 430F (220 c). Corrosion inhibitors are totally ineffective in these conditions.

In sour systems, there are three generally recognized levels of corrosion resistance and high molybdenum alloys such as alloy 625 - with 9% Mo is generally credited with being able to handle the most severe conditions, including a presence of free sulphur which can act as an oxidiser. Nickel plays an important role in the prevention of stress corrosion cracking in the presence of chlorides and the combination of nickel and molybdenum is very effective against sulphide stress cracking.
What are the critical weld overlay process factors to control...?
The main consideration with the surfacing process is achieving correct composition of the surfacing material. Selection of the most appropriate alloy is paramount but the amount of parent metal melted and mixed in with the filler metal (the degree of dilution) is also of crucial importance. This is generally expressed as a percentage dilution of parent metal in the surfacing.

Dilution varies from process to process and is influenced by welding parameters, in particular electrode polarity, welding current and travel speed. These need to be closely controlled to achieve consistency.

The composition of the surfacing alloy is selected for the specific application, ranging from tungsten carbide rods or martensitic steel wires for wear resistance, to austenitic stainless steels for corrosion resistance, and from nickel alloys for oxidation resistance to copper-nickel alloys for saltwater service.
What are the Economic arguments in favour of Corrosion resistant alloys...?
Corrosion resistant alloys have traditionally been used in oil & gas production as a last resort when protective coatings, cathodic protection or corrosion inhibitors have proved inadequate, for example in the development of deep sour gas wells, CO2 enhanced oil recovery and with in-situ fireflooding operations.

Important factors in project evaluation are production rate, operating costs, maintenance costs and capital costs. Several economic studies indicate that the use of corrosion resistant alloys is readily justifiable for depths over 15,000 feet. Below that level corrosion resistant alloy systems require more capital outlay than inhibition systems which however require more maintenance and more expense over the lifetime of a gas well.

Corrosion resistant alloy systems are most economically attractive when installed initially, not as a response to inhibition system failure, and also when installed as a complete well system.

For a 20 year design life, corrosion resistant alloys should be considered in applications where corrosion rates greater than 0.2 mm/yr are projected.
What causes corrosion...?
The role of corrosion resistant alloys for deep gas wells has become well established, through better understanding of the corrosive behaviour of hydrogen sulphide, carbon dioxide and seawater environments at high pressure and temperature, and the performance limitations of certain materials in these conditions, translating into improved operation and high equipment reliability.

In many offshore applications, corrosion resistant alloys can be economically justified as an alternative to carbon steel with continuous corrosion inhibition. System repairs often run into millions of dollars and it is therefore desirable to decide on the use of corrosion resistant alloys tubulars for downhole, sea water and product piping systems in the early development of an offshore field, especially in deeper waters.

Sour crudes, containing considerable quantities of H2S, CO2 and salt water, show a high general corrosivity, localised pitting & crevice corrosion, and to sulphide and chloride stress corrosion cracking, or through combined action.

Corrosion resistant alloys have traditionally been used in oil & gas production as a last resort when protective coatings, cathodic protection or corrosion inhibitors have proved inadequate, for example in the development of deep sour gas wells, CO2 enhanced oil recovery and with in-situ fireflooding operations.
What is weld cladding...?
Cladding is defined by the Welding Institute as; providing a corrosion or oxidation resistant surface on a less corrosion resistant material.

An example of this is the deposition of a stainless steel or nickel-based layer on a carbon steel base. One advantage of this technique is cost saving arising when surfacing a relatively inexpensive metal, such as a carbon steel, with a more expensive but corrosion resistant layer of stainless steel.

Material and weight savings may be gained when a clad, high strength, quenched and tempered steel is used in a corrosive environment.

IODS use as range of alloys to offer weld cladding services for process flow and control systems, providing long-life, high-reliability corrosion resistance.
When was weld overlay cladding developed and how is it produced...?
Surfacing techniques have been used in a variety of applications for thousands of years but it is only since the 1940s that arc welding has been used. Since that time, all of the arc welding processes have been employed.

Various welding techniques have been adapted to overlaying and the selection of the specific technique is dependent upon access, welding position, alloy type and dissolution rate, and economics.

The ability to most economically achieve required alloy composition at the required deposit thickness depends on these factors, and there is a trade-off. For example, a high deposition rate may appear fast and therefore labour cost saving, however if the heat input to achieve this rate is too high then excessive dilution with the underlying base material may mean that a second layer is required. Weld overlay processes can be used on moderately thin components although it may be necessary to provide some heat sink removal of heat during the welding process.

In general, vessel cladding by weld overlaying may be considered above 30-50mm thickness, but is most appropriate for heavy wall thicknesses. It is also selected in some cases due to easier availability compared to a mill product.

With correct process control the correct chemical deposition can be achieved in one layer, proving an economic solution. Heat treatment may be required after overlaying if the heat affected zone (HAZ) hardness of the backing steel exceeds specified limits. Dependent on the backing steel type, a tempering treatment may be sufficient to soften the HAZ without affecting the corrosion resistance of the overlay.

The preferred weld overlaying methods are usually pulsed or synergic Gas Metal Arc welding (GMAW) and hot or cold wire Gas Tungsten Arc Welding (GTAW). These processes are suitable for positional welding and can be carefully controlled to meet dilution specifications. GTAW is the most easily adapted for small bore or difficult area access.

For pipe weld overlay cladding applications, the limit on the length which can be weld overlaid is determined by the rigidity of the welding torch.
Where is weld overlay cladding used...?
Every sector of industry - oil and gas, automotive, aerospace, power generation, yellow goods etc - uses arc surfacing techniques for repair and recovery and to improve service performance.

Clad plate for subsequent forming into vessels was produced by weld overlaying in the early development days.

Weld overlaying is now directly applied to the completed vessel shell. Overlaying of heavy vessels was originally developed for the nuclear vessels, oil refinery hydro-cracker vessels and pulp digesters, with increasing application in the Oil and Gas sectors for separators, heat exchangers etc..

Weld clad components and pipes can be used as a cost-effective solution to corrosion prevention in any environments where highly corrosive agents are likely to occur, and where the environmental and commercial impact of system failure is un-acceptable.
Why is corrosion resistance important in the Oil & Gas industries and where can weld overlay be applied to solve corrosion problem...?
An increasing share of world Oil and Gas production is now offshore and improved technology allows access to deposits in increasingly deep water and severe environments.

There is a need for high reliability and low maintenance costs and a key approach developed over the last 10 years is to use corrosion resistant alloy materials. The corrosive environments which favour use of corrosion resistant alloys are seawater, oil and gases which contain carbon dioxide and/or hydrogen sulphide.

As offshore platforms become bigger and more sophisticated, moving into deeper waters, there is a requirement to reduce platform weight and optimise deck space. Weight saving is therefore a key benefit, and studies have shown that a reduction in top weight of 1 ton allows a corresponding 3 ton reduction in supporting structure weight, giving a $150,000 cost saving per ton. The total weight of seawater piping systems on a single platform may be several thousand tons.

There is considerable variation within the industry in the assessment of corrosivity of oil and gas process streams, criteria for acceptance of corrosion rates, attitude to inhibitors vs corrosion-resistant materials, emphasis on initial cost rather than total cost and so on.

However, in the last few years there has been growing interest in the use of alloy-clad piping for risers, flow lines and subsea template piping/manifolds. Potential problems related to cathodic protection of stainless steel have tended to encourage this interest, also stimulated by controversy over resistance of duplex stainless steel to sour conditions.
Why use IODS for corrosion resistance weld cladding solutions...?
IODS deliver complete pipe system corrosion resistant solutions.

Our Oiltool division offers weld cladding services for individual components - either issued by you, or sourced and supplied by us.

Our Pipeclad division supplies weld clad pipe products and complete pipe systems. We source the pipe to your specification, clad the pipe and manage the order to your delivery timescales.

Our integrated supply chain allows us to deliver against tight schedules and we project manage your project from initial scoping, through procurement and production to final order assembly and ship, with staging and final on-site assembly if required.

Our commitment to quality and our investment in product NDT assures that we continue to deliver high reliability long life solutions for your harsh environment corrosion resistance applications.


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