Materials

Steel and aluminium are the materials mostly used for the fabrication of anchors and anchor chains. These materials are cheap, have a good strength-to-weight ratio and they are wildly used in the fabrication of other industrial goods. So a very profound long-term experience on strength, corrosion and metal weakening is available.

Steel

Steel is a combination of iron and carbon additionally alloyed with various elements (Chromium, Nickel, Silicon, ...) to improve physical properties such as hardness, ductility or tensile strength and to produce special properties such as resistance to corrosion, wear and abrasion or heat.
Steel has been used since centuries and a large variety of different types have been developed each with specific physical, chemical and mechanical characteristics.

Stainless Steel

Stainless steels are high-alloy steels that have superior corrosion resistance because they contain large amounts of chromium. Stainless steels can contain from 4 to 30 percent chromium, but typically around 10 percent.
Stainless steels can be divided into three basic groups based on their crystalline structure: austenitic, ferritic, and martensitic. Another group of stainless steels known as precipitation-hardened steels are a combination of austenitic and martensitic steels. Below are the general compositional contents of these groups.

Grades of Stainless Steel

• Ferritic grades: Ferritic stainless steels are magnetic non heat-treatable steels that contain chromium but not nickel. They have good heat and corrosion resistance, in particular sea water, and good resistance to stress-corrosion cracking. Their mechanical properties are not as strong as the austenitic grades.

• Martensitic grades: Martensitic grades are magnetic and can be heat-treated by quenching or tempering. They contain chromium but usually contain no nickel. Martensitic steels are not as corrosive resistant as austenitic or ferritic grades, but their hardness levels are among the highest of the all the stainless steels.

• Austenitic grades: Austenitic stainless steels are non-magnetic non heat-treatable steels that are usually annealed and cold worked. Some austenitic steels tend to become slightly magnetic after cold working. Austenitic steels have excellent corrosion and heat resistance with good mechanical properties over a wide range of temperatures. There are two subclasses of austenitic stainless steels: chromium-nickel and chromium-manganese low-nickel steels. Chromium-nickel steels are the most general widely used steels and are also known as 18-8(Cr-Ni) steels.
  The chromium nickel ratio can be modified to improve formability. The carbon content may be reduced to improve inter-granular corrosion resistance. To improve corrosion resistance, molybdenum can be added and additionally the Cr-Ni content can be increased.

In today's terms stainless steel is used as a generic term to describe corrosion resistant steel that has a minimum chromium capacity of 10.5 percent. The chromium creates a passive and self renewing chromium oxide film around the steel at atomic level and this prevents the iron from rusting.

Galvanized Steel

As a corrosion resistant alternative to stainless steel, galvanized steel can be used. It is available in the same strength as stainless steel at lower cost. But in order to keep the corrosion under control, it must be examined regularly and re-galvanized if necessary.
Galvanized steel is coated with zinc, which protects steel in two ways:

• First, it is highly resistant to rust; iron, a major component of steel, reacts very easily with oxygen and moisture and will eventually disintegrate. The layer of zinc on the surface prevents those elements from reaching the steel so quickly. It also develops a patina - a layer of zinc oxides, salts, and other compounds - that offers further protection. Zinc is also extremely durable and scratch resistant.

• The outer zinc layer also protects the steel by acting as a "sacrificial layer." If, for some reason, rust does take hold on the surface of galvanized steel, the zinc will get corroded first. Even in areas where the surface is scratched or damaged, the surrounding zinc will still corrode before the steel does.

For marine applications, hot dipped galvanisation gives best results: the steel is immersed in a bath of molten zinc and coated to a significant thickness. Hot dip galvanizing may change the temper of the steel but unlike to what is often written, it does not change the structural strength and it can be applied also on HT steels (e.g. galvanized HT grade 40 steel).
The zinc coating serves as sacrificial coating and will dissolve after some time. When the zinc coating is dissolved, the underlying chain will start to rust. So as soon as rust appears, anchor and anchor chain must be re-galvanized, else they start losing strength and weight. Before re-galvanization it is very important that all rust is thoroughly removed. Take into account that after re-galvanisation the strength of the chain or anchor will be reduced by about 10 to 15%.

Aluminium

Aluminium is a relatively light metal compared to metals such as steel, nickel, brass, and copper. Its specific weight is about 2700 kg/m3, which is only one third of the specific weight of steel. It also has good electrical and thermal conductivities and is highly reflective to heat and light.
Aluminium alloys have a strong resistance to corrosion which is the result of an aluminium-oxide skin that forms as a result of oxidation reactions with the atmosphere. This corrosive skin protects aluminium from most chemicals, weathering conditions, and even many acids, however alkaline substances are known to penetrate the protective skin and corrode the metal.

The advantage of aluminium is its good strength at low weight. However, it is never used as chain (probably because of it's light weight, anchor chain requires weight under water to keep the departure angle of the chain low) .. ?
Aluminium anchors (e.g. "Fortress") are more comfortable to handle and are often used as backup anchors, which are usually kept in one of the cockpit lockers and must be heaved out for use.

Strength Characterization

The characterization of strength of materials is an important concept in material science and also in mechanical and structural engineering. The tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before failure. However, the definition of failure can vary according to material type and design methodology.
There are two typical definitions of tensile strength:

• Yield strength, or the yield point, is defined as the stress at which a material begins to plastically deform, and will show permanent deformation after the stress has been removed. In order to define this point more precisely, the amount of residual deformation must be defined. Yield strength is the stress which will cause a permanent deformation of 0.2% of the original dimension. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permanent and non-reversible.

• The ultimate strength is the maximum stress a material can withstand at the point of breaking. For steel and aluminium alloys, the ultimate strenght is about 50% higher than the yield strength.

The wide ranges of yield strength and ultimate strength are largely due to different heat treatment conditions.

Since methods used to classify steel products vary considerably throughout the world, the global steel industry recommended a classification system that defines both yield strength (YS) and ultimate tensile strength (UTS) for all steel grades. In this nomenclature, steels are identified as "XX aaa/bbb", where:
XX = Type of steel; aaa = Minimum YS in MPa and bbb = Minimum UTS in MPa.
For example, in this classification system DP 500/800 refers to dual-phase steel with 500 MPa minimum YS and 800 MPa minimum UTS.
Note that tensile strength depends on temperature. However, in the range of normal environmental temperature, the tensile strength of steel and other metals can considered to be constant.

Electrolytic Corrosion

The galvanic difference between stainless steel and regular steels (e.g. galvanized steel) is large enough to start electrolytic corrosion when these materials are mixed in a system (e.g anchor system, windlass, winches, ...) in which different materials may be contacted by salty sea water.

Marine Applications

The salt water environment in which the ground tackle components are used, exposes the material heavily to chemical corrosion through oxidation and electrolysis. Steel can be made more resistant to oxidation (rust) and corrosion by protecting the surface against the corrosive environment. This can be done by adding e.g. chrome to the steel. The chrome on the surface of the steel will oxidise forming an impenetrable layer effectively blocking oxygen from penetrating the steel. Even when the oxidation layer is scratched, the chrome available in the steel will rebuilt the chrome-oxide layer.
Another technique is to cover the surface of the steel with a less nobler (e.g. zinc) metal, which will corrode more preferably than the steel. In this case the protective layer should be thick enough to maintain protection in case it is scratched. Eventually, the protective layer will have to be renewed after some time.
Although, stainless steels have higher resistance to oxidation and corrosion in many environments, it is important to select the correct type and grade of stainless steel for the particular application.

Even for specialized marine applications, at least three types of steel are in use:

• "marine grade" stainless steel
  e.g. type 316L

• zinc-coated hot dip galvanized steel
  e.g. type Grade 30 "Proof Coil" or Grade 40 "High-Test"