Factors affecting the selection of Austenitic (316

grade) or Austenitic-Ferrite mix (duplex) Stainless

Steels in their application in black-water pipework

Introduction

Material selection for pipework is largely conducted according to recognised

specifications and well understood factors. In the marine environment, the selection

is still largely based on existing criteria that have proven to work in onshore

industries, with one notable exception – smoke toxicity. This brief non-technical

article will look into the selection factors between 316 grade and Duplex stainless

steels, so smoke toxicity is not relevant.

Selection Criteria

A number of factors are involved in the evaluation and selection of materials for

pipework, and are largely dependent on the anticipated operating conditions during

the service life. The following factors should be considered:

Intended use – Type of wastewater

Scour or abrasion conditions

Installation requirements

Corrosion conditions – chemical, biological

Flow requirements – pipe size, velocity, drag

Product characteristics – pipe size, length, fitting and connection

requirements

Cost effectiveness – materials, installation, maintenance, life expectancy

Physical properties - stiffness, loading strength, stress and flex

Handling requirements – weight

This selection list gives a broad outline of the many different factors that needs to be

considered. How each one is evaluated or weighted against each other depends on,

to a large extent, the operating environment; an operational ship environment poses

significant design and operational problems that are not often matched in onshore

installations. E.g. the pipe layout has to take into account the complex ship design

and compartment layout, resulting in many and varying changes in the direction of

pipework. Weight is always a significant consideration and the combined weight of

the entire black water pipework adds a sizable load to the above waterline weight.

Corrosion, whether it’s initiated by chemical, biological or physical conditions, is a

persistent problem with steels (in varying amounts), and is particularly prone to this

form of damage. Polymeric materials are inert to corrosion, and while they would

make ideal materials for pipework, their smoke toxicity and poor wear characteristics

seriously hamper adoption.

Chemically induced corrosion occurs when the pH of the wastewater moves away

from pH 7 (neutral) and remains in contact with the pipe material for an extended

period of time. The corrosive nature of the ions in solution attack the protective oxide

surface layer and expose the underlining steel to the environment.

Dissolved oxygen and certain anions, namely the halogens (chlorides, bromides) all

contribute to the initiation and acceleration of corrosion. Chlorides (typically from sea

water or chloride contain acids) are particularly effective at initiating corrosion in

steels, leading to the formation of many tiny highly localised points or ‘pits’ of

corrosion, called ‘Pitting’i. Pitting is a dangerous form of corrosion attack for several

reasons. Pits can result in the perforation of a metal component while the rest of the

metal piece remains unaffected. In the presence of an applied stress, pits can serve

as sites to initiate stress corrosion cracking, leading to complete failure.

Biological induced corrosion is predominately through the contamination of sulphate-

rich wastewater with Sulphate Reducing Bacteria (SRB). SRB’s are a group of

bacteria that use Sulphates dissolved in water to aid their respiration and

metabolism, rather than using oxygen, this process is called Anaerobic respiration.

The bacteria make their home on the interface between the steel surface and the

water, and through a complex metabolic process reduce dissolved sulphates (SO42-)

to hydrogen sulphite gas (H2S), a highly toxic and corrosive gas that attacks the

metal.

i McCafferty, E. (2010) Introduction to Corrosion Science, Springer: First Edition, pp. 272-285

The flow characteristics of moving water can initiate corrosion through the abrasive

action of turbulent flow, cavitation and impacts of solids against the inner surface.

Current pipework design and layout can mitigate many of these problems, but the

strength and corrosion resistance of the steel still forms a significant consideration.

Properties of 316-Grade and Duplex Grade Stainless Steelii

Type 316 is an austenitic chromium-nickel stainless steel containing molybdenum.

The addition of molybdenum increases general corrosion resistance, improves

resistance to pitting from weak chloride ion solutions (limited resistance), and

provides increased strength at elevated temperatures. Corrosion resistance is

improved, particularly against sulphuric, hydrochloric, and other organic acids.

However, they are susceptible to stress corrosion cracking (SSC). E.g. 316 is not

used in and around swimming pools due to its particular susceptibly to SSC by the

chloride ion (hypochlorite disinfectant used to treat the water).

Type 316 grades are considered to be readily formable and weldable by common

practices.

Duplex Stainless steel grades, which are a balanced mix of Austenitic-Ferrite

components, have a number of advantages over 316 grade steels:

Higher strength leading to weight saving

Greater corrosion resistance particularly SSC

Lower price

Better price stability

Of the four the first one is the most significant and advantageous over 316 grade;

Duplex is around twice the strength of 316 and therefore half the thickness is

required for the same anticipated forces. However, that advantage soon turns to a

major disadvantage as high strength results in formability and machinability issuesiii –

ii http://www.bssa.org.uk/topics.php?article=668

iii Practical Guidelines for the Fabrication of Duplex Stainless Steels, IMOA: Second Edition

(2009),

http://www.imoa.info/download_files/stainless-steel/Duplex_Stainless_Steel_2d_Edition.pdf

Duplex grades are less ductile than 316 and therefore shaping, bending and

machining become significantly more complex, if it can be achieved at all.

Where higher corrosion resistance is required, alongside resistance to SSC, Duplex

can handle many different corrosive environments and conditions, from: concentrated

acids and alkalis, hot and cold liquids, Chloride ions and prolonged contact with

corrosive chemicals.

Conclusion

Duplex grades demonstrate remarkable versatility over 316 grade steels particularly

when required to operate under corrosive and/or high stress conditions. However, as

noted, the high strength characteristic of Duplex becomes a disadvantage when

formability and machinability is taken into account – this can be a significant cost

effectiveness problem.

The question is, is the enhanced corrosion resistance and higher strength

characteristics of Duplex over 316 grade justifiable for the conditions expected to be

found in the black water lines? If they continue to use Chloride acids (hydrochloric

acid) to provide periodic descaling without a suitable corrosion inhibitor, then a case

can be made. Although, in the absence of material and installation costings it is

difficult to make an assessment as to whether Duplex offers an enough of an

advantage over 316.