Page 264 - Handbook of Adhesives and Sealants

232 Chapter Six

6.5.1.1 Chemical treatment of metal surfaces. The purpose of chemi-
cally treating a metal surface is generally either to remove an un-
wanted oxide or other weak boundary layer, or to protect the surface
from corrosion. Chemical treatments also provide surface roughening
on a microscale which improves mechanical adhesion. Chemical treat-
ment processes generally remove the complex elements that are on
the substrate surface and replace them with a more uniform, more
tightly held structure.
Metal surfaces are usually some combination of oxides, sulfides,
chlorides, acid salts, absorbed moisture, oil, and atmospheric gases.
These weak boundary layers are formed by the process used to fab-
ricate the metal, and their surface characteristics are dependent on
the parameters of these processes. The resulting surface structure is
usually weakly bonded to the base metal and prone to crack or flake
off. With many metal substrates, it is not sufficient to only remove
these weak layers. The pure, bare metal surface may be very reactive,
and unwanted oxide layers and corrosion products could quickly form.
Thus, the surface preparation must not only remove the original sur-
face, but replace it with a surface coating that will protect the inter-
face during further processing and during the joint’s service life.
The best method of removing surface material is by mechanical pro-
cesses described previously. However, chemical pickling or acid des-
caling are chemical methods to remove mass surface material. With
these processes, the metal oxide surface is effectively and rapidly dis-
solved leaving bare bulk metal. Pickling may employ a dip or spray
system. Hydrochloric, sulfuric, and nitric acids are often used. The
particular acid will depend upon the metal and the type of oxide being
treated. The rinsing and drying of the substrate, once pickling is com-
plete, is a very important step so that all acid and acid by-products
are completely removed from the surface. It is important to use inhib-
ited acid cleaners to avoid corrosion of cleaned surfaces. A limitation
of acid descaling is that some dimensional change will occur in addi-
tion to oxide removal.
Chemical etching processes are similar to pickling except that a
more complex solution is generally used. This surface preparation pro-
cess not only removes surface layers, but it also transforms the sur-
face, making it more chemically active and more receptive to bonding
with certain adhesives. Common etching processes for aluminum al-
loys use chromic–sulfuric or phosphoric acids. Sulfuric acid based etch-
ing processes are commonly used for steel. These multi-stage processes
require careful control, and safety and environmental issues are often
difficult.
Corrosion of many metal surfaces begins immediately after removal
of the protecting oils or contamination. Corrosion can quickly affect