When working with wood, it is common to use all types of anchors. The most common are screws, but also nails, clamps, etc. All of these can be affected by the environment, especially outdoors and / or coastal or maritime areas.

Therefore, it is necessary to choose the appropriate anchor, material and protection, if we want our wooden elements or structures to remain in good condition.

In some cases, the use of certain types of wood can even accelerate the deterioration or corrosion of screws and other metal anchors. Some of these species are cedar, teak or walnut.

Problems Caused by Corrosion

Corrosion of wooden fasteners can cause approximately the following types of problems:

Aesthetic problems that manifest themselves in a short period of time: superficial black stains on the screws or oxidation of visible connection systems, wood stains …

Structural problems: rusted connections (in this case screws, nails …) continue to rust until they cannot guarantee the static resistance with which they were placed in place.

In the search for solutions to the corrosion problem, low mechanical strength fixings; this can cause:

  • Screw breakage during insertion.
  • Screw breakage due to movements of the wood (it is often not a problem of the screw, but the fixation of the wood).

Classes of service

Not all situations require the same solution, so it is necessary to classify the conditions to which the screw, nail, fixation, etc. will be subject …

Class of service 1. Inside. It is characterized by a moisture content in the wood corresponding to a temperature of 20 ± 2 ° C and a relative humidity of the air that exceeds only 65% ​​a few weeks a year. In most conifers, Humidity <12%.

Class of service 2. Indoor exterior. It is characterized by a moisture content in the wood corresponding to a temperature of 20 ± 2 ° C and a relative humidity of the air that exceeds only 85% a few weeks a year. In most conifers, Humidity <20%.

Class of service 3. Outdoor uncovered without contact with the ground. Environmental conditions that lead to a moisture content higher than service class 2. In most conifers, Humidity> 20%.

Solutions to the corrosion problem

There are basically two options for solving the corrosion problem in steel fasteners and / or screws:

  • Protecting steel with suitable coatings.
  • Plating.
  • Paintings.
  • Use stainless steel.

Plating

Galvanization is the electrochemical process by which one metal is coated on another. In the present case, zinc is used mainly, and sometimes in combination with chromium.

Why zinc? Because it is less noble than iron and under normal environmental conditions, it has a lower corrosion rate. In addition, its corrosion products are less showy than red iron oxide.

The resistance of the coating depends on the environmental characteristics and the duration of protection depends on the thickness.

Advantage:

  • Slow and uniform corrosion in the atmosphere.
  • Possibility of passivation.
  • Barrier and active protection.
  • Low cost.
  • Possibility of using materials with better mechanical properties compared to stainless steel.

Disadvantages

  • Surface finish.
  • Limited duration.
  • It is necessary to replace Chromium 6+ (toxic component).

Stainless steel

A steel is defined as stainless if it contains at least 12% chromium, an element that allows the formation of a passive oxide film. Another premise for the formation of the oxide film is the presence of an oxidizing environment that allows its formation.

Stainless steel has an oxide layer of continuous coating, without porosity, compact and capable of self-healing (passive behavior).

Advantages and disadvantages of using stainless steel

Advantage

  • Safety: excellent resistance to corrosion (austenitic> ferritic> martensitic).
  • Aesthetics: shiny surface.
  • Good temperature resistance (chromatization guarantees protection up to 80ºC).
  • It does not need to be coated (there are no allowable thickness problems for the coupling).
  • Simple maintenance (no rust or deposits, no unscrewing problems, no friction
    due to oxidation).

Disadvantages:

  • Lower mechanical properties (martensitic> austenitic> ferritic).
  • High cost.
  • Corrosion by corrosion (perforation; it is a problem of stainless steel that produces micro holes in the material).
  • It is incompatible with other metals such as brass or copper, or even with zinc or aluminum in marine environments due to the presence of salt. This can have implications when working with wood if, for example, we combine incompatible metal screws with steel clamps to secure a pallet to the slats. Cases like this, or equivalent, are unfortunately common.

Wood-based recommendation

Stainless steel A4 It is the most noble to resist corrosion of any kind, but its disadvantage is its low mechanical resistance (pre-drilling is recommended when assembling the work) and a very high cost.

A2 stainless steel It is standard stainless steel with good corrosion resistance (not ideal for the marine environment due to the pitting phenomenon), but it has the disadvantage of low mechanical resistance (pre-drilling is recommended during installation).

AISI 410 It is a stainless steel with low corrosion resistance (ideal for less aggressive environments) but it has a very good mechanical resistance (it can be installed without pre-drilling in very dense wood; with very dense wood, pre-drilling is always recommended due to a problem of breaking Madeira). The cost is less than the other two.

Coated Carbon Steel The specific protection can be approved up to use class 3, but it is not recommended for facades because they can cause aesthetic problems with black spots.

Carbon steel with electrolytic galvanization it is generally approved for use in class 2.