There are three common finishes for steel fixings and fasteners; BZP – Bright Zinc Plated, HDG – Hot Dipped Galvanised and Stainless Steel. With Stainless Steel typically split between A2 and A4 types. In this blog post we summarise each finish and try to highlight the key differences between them.
Bright Zinc Plated and Galvanised steel are the most common types of fixing finish, are easy to work with and offer a lower cost option than stainless steel. Whereas although it has a higher cost and is potentially harder to work with, stainless steel offers the advantages of strength and increased corrosion protection, as well as providing a quality finish that’s aesthetically pleasing.
For ease we will focus on comparing BZP and HDG and then types A2 and A4 Stainless steel, before providing a summary of considerations for choosing each one.
BZP – Bright Zinc Plated
BZP stands for Bright Zinc Plated and is the most common finish for the majority of fixings and fasteners. Bright Zinc Plating is a process that is applied to standard steel or alloy fixings during their manufacture. Essentially a thin layer of zinc is applied to the fixings using an electric current. The applied layer of zinc then offers galvanising protection to the steel fixings to prevent rusting or oxidising. Although the BZP finish makes the steel fixings resistant to moisture, the layer of zinc is only thin so it won’t prevent rusting or oxidising if the fixings are used externally. Hence BZP is a good choice for indoor applications.
As BZP is a standard low cost finish, freely available for most types of fixing, specifying it can help keep all the fixings used on an installation uniform. A BZP finish also has the advantage of being aesthetically pleasing, with the fixings used looking bright and shiny.
HDG – Hot Dip Galvanised
HDG stands for Hot Dip Galvanised and refers to a process where the finished fixings and fasteners are dipped in a bath of molten zinc which bonds with the steel to provide a protective layer (thicker than BZP plating). Hot Dip Galvanising offers a cost-effective method to provide a strong weather resistant finish which protects steel from corrosion. It’s important to consider that when threaded fasteners are Hot Dip Galvanised, the size of the thread increases due to the protective layer of zinc. So, you’ll need to use a set of HDG fasteners for all parts of the fixing. HDG fasteners have an oversize tap which accommodates the extra layer of zinc allowing each fastener to fit together neatly. For example, the use of HDG nuts should be used with HDG bolts and HDG threaded rod.
Although duller in appearance and not as aesthetically pleasing as other finishes such as BZP, with its strong corrosion protection galvanised fixings offer the advantage of being low maintenance once installed and can be used for outdoor applications. One final consideration is that if the protective zinc layer is damaged such as when HDG products are cut, then a galvanising spray should be used to cover the exposed area in order to continue providing protection.
Choosing Between BZP and HDG
Both BZP and HDG add a protective layer of zinc to steel although typically a BZP Zinc plating will be 3 microns thick and the HDG protective Zinc layer will be 50 microns thick, hence the far stronger protection offered by galvanised fixings. So, when choosing between the two finishes, it’s reasonably simple; BZP fixings are great for indoor dry climates where only superficial protection is needed and an element of aesthetic value is desired. Whereas, HDG fixings should be used for outdoor applications open to the weather and also indoors where conditions may be damp or wet.
A2 and A4 Stainless Steel
Stainless Steel is known for its strength and resistance to corrosion. All Stainless Steel must contain at least 10.5% chromium, which helps the steel resist corrosion. When used for fixings and fasteners Stainless Steel is covered by the British Standard BSENISO 3506, in Parts 1, 2 and 4 (Part 1 covers bolts, screw and studs, Part 2 nuts and Part 4 tapping screws).
Stainless steel differs from standard carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. Stainless Steel contain sufficient chromium to form a passive film of chromium oxide, which helps prevent surface corrosion and corrosion from spreading into the steel’s internal structure.
Although both HDG and Stainless Steel fixings are decent options to protect against rust, Stainless Steel offers greater corrosion protection than HDG and is ideal for use in outdoor applications, especially where the finish is desired to be aesthetically pleasing. There are two main grades of Stainless Steel fixings; A2 Stainless Steel (Type 304) and A4 Stainless Steel (Type 316) which are both very rust-resistant grades of stainless steel. There is no evident visual difference between A2 and A4 stainless Steel fasteners as they are both polished using the same process, but they do have slightly different characteristics.
A2 Stainless Steel (Type 304)
A2 Stainless Steel contains 18% Chromium, and 8% Nickel to provide corrosion protection. A2 Stainless Steel is the most common type used for the manufacture of Stainless Steel fasteners and fixings. Aside from fasteners, A2 Stainless Steel is also used for many household products due to its resistant to sterilising solutions, foodstuffs and inorganic chemicals. A2 Stainless Steel fixings can be used by MEP and HVAC contractors in outdoor applications or where Galvanised fixings aren’t suitable. Although even A2 Stainless Steel can tarnish eventually over time.
A4 Stainless Steel (Type 316)
A4 Stainless Steel typically contain 18% Chromium, 10% Nickel, with an additional 2-3% Molybdenum too, which provides extra corrosive resistance than A2. Since it has the highest level of corrosion resistance, A4 Stainless Steel is often used in chemical or marine environments hence it can be referred to as ‘Marine grade’. A4 Stainless Steel products generally last a very long time due to their extra resistance to corrosion and tarnish.
Choosing Between A2 and A4 Stainless Steel
As you’d expect A4 Stainless Steel costs slightly more than A2 because of its enhanced properties. Although, the extra cost can be justified when used in the right conditions, with the higher corrosion resistance meaning over a long period of time the fasteners need less maintenance or replacement. Both types can be a cost-effective option, with A2 Stainless Steel fasteners being far the most commonly used in construction. However, when choosing which type of Stainless Steel fasteners to go for, essentially MEP and HVAC contractors should consider the environment they will be used in. A2 Stainless Steel fixings will be up to the vast majority jobs including those near to marine environments subject to specification. However, if you are using fixings in a specific marine or chemical environment which require industrial-strength corrosion resistance, then A4 Stainless Steel fixings should be used.
Which Type of Steel Finish is Best?
As you would have read above there are pros and cons for each… BZP and HDG steel fixings are generally cheaper and more readily available, with HDG fixings offering very good corrosion protection. Cost and availability can obviously be large factors in any decision, but other considerations where Stainless Steel comes out on top such as strength, corrosion protection and the end quality / aesthetically pleasing nature of the finish should be considered too. Ultimately there is no definitive answer and a lot will depend on the conditions the fixings will be used in and project specifications such as; budget, supply, continuity of finish, expected lifespan, ease for maintenance or replacement, indoor or outdoor, wet or dry conditions, possible contact with chemicals etc. By considering each of these elements the right type of steel finish can be chosen to suit the required project.
Steel Finishes Summary
BZP – Bright Zinc Plated:
Advantages:
- Low-cost option
- Aesthetically pleasing finish
- Easy to work with
Disadvantages:
- For indoor use, as low corrosion resistance
HDG – Hot Dip Galvanised:
Advantages:
- Relitively low-cost option
- Offers good degree of corrosion protection
- Easy to work with
Disadvantages:
- Dull look, not as aesthetically pleasing as the other finishes
A2 Stainless Steel:
Advantages:
- Offers high degree of strength and corrosion resistance
- Aesthetically pleasing finish
Disadvantages:
- Higher cost option (although depending upon conditions can be a very cost effective by reducing maintenance and replacement)
- Due to strength can be harder to work with when installing
- Can tarnish over time
A4 Stainless Steel:
Advantages:
- Offers highest degree of strength and corrosion resistance
- Can be used in marine and chemical environments
- Less likely to tarnish over time
- Aesthetically pleasing finish
Disadvantages:
- Highest cost option (although depending upon conditions can be a very cost effective by reducing tarnishing, maintenance and replacement)
- Due to strength can be harder to work with when installing
- Supply – not so freely available, as for use in specific environments
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